The only way to afford spending gold is by raiding barbarians, so increasing a little bit the density of the barbarians can help.
In my experience there are a few other ways to break even on money, without needing barbarians.
1> The Wealth process, which converts production into gold. Almost never gets used in the vanilla game, but it's actually very handy; once you get Factories and such, or once you hit Nuclear Fission and your mines' production increases, then the ratio is good enough that you might just want to set many core cities to this, instead of using the old build-and-disband cheat.
2> Improve the base gold of your cities. The new national treasury wonder thingy, a larger emphasis on Trading Posts, using Great Merchants to make a Customs House, and so on. The Customs House, especially, becomes VERY valuable in the long term, and I'm working right now on a change to make it even more so.
3> Start a Golden Age (which ties back to happiness and great people). Even if you can't break even during a normal turn, you'll make a nice profit in the GAs. In my last game I'd lose 20 gold a turn on a normal turn but make +100 per turn in a GA; by careful use of my leaders, I did just fine.
4> Make sure you're not building every building you possibly can; just because you CAN build that Opera House in your low-culture city doesn't mean it's a smart idea to. When you conquer a city, go through and delete whatever you can't afford/don't need (especially Harbors and Barracks).
The only buildings that are absolutely no-brainers to keep/build are those with no maintenance, like the Markets or City Walls. For culture, happiness, and research, the first tier or two of buildings (Monument, Temple, Colosseum, Library, maybe University) are also definitely worth the cost, but beyond that you have to start considering the balance, because the ratios get worse. So while your core cities should definitely look into the Theater, Stadium, Opera House, Museum, and the various military buildings, they're no longer no-brainers for EVERY city.
This also becomes very pronounced in the future eras, since I've added several buildings with negative happiness (most notably the Children's Creche (-1) and Genejack Factory (-2)). You start having to seriously consider whether it's worth building all of those; the answer is almost always yes, but it's not automatic.
5> Plan your trade routes out better, and take your time getting new cities linked up. Don't make a massively long road connection; build some Harbors instead. (This is a big advantage for the Iroquois.) If a new city is 6 hexes away from its neighbors, and you connect them with a railroad, then the new city will need to be size ~7 before you even break even on the land trade route, barring any wonders or policies that tweak this.
Obviously a land route has other benefits, but you get what you pay for.
6> Sell resources to the AIs for cash instead of asking for a resource in return. The AI's pretty bad about hooking up multiple deposits of a luxury, so in some games you have no choice but to do this early on. But in my last game I had 5 Cotton, so I'd sell the extras to various civs for ~300 gold apiece for a 30-turn agreement. That's +40 gold per turn from the spares, which allowed me to run a deficit normally.
Obviously, giving the AI players more happiness will hurt you in the long term, so it's not money for nothing. But if you're careful about this, and only trade to the weaker AIs, this can help you stay afloat during the tough stretches.
By the late game, once you have a decent-sized empire, you'll have plenty of money. I'm still working on the balance at that end; in my last game (Ancient start, game stalled in the Nuclear for technical reasons) I was getting +150 gold per turn in a normal turn, +400ish in a golden age.
I've a problem regarding The hanging Gardens, the bonus is +1 population in each city, it's fine for vanilla, but with your mod it's suicide, should give happiness as well, at least as much as unhappiness it causes to balance itself out.
The difference between vanilla and my mod is slim for this; vanilla is 1 unhappiness per population, I'm 1.2. The only difference is that in vanilla you often have a much bigger happiness buffer to work with,
and that was one of the reasons I changed this in the first place. But I've seen this same scenario play out in vanilla; you build the Hanging Gardens and suddenly drop into severe unhappiness. The AI never has to worry about that because they'll almost always have a massive happiness built up, while the human will settle new cities to stay near 0. But there's no reason you can't delay building that new city if you're getting close to building the Gardens. (And that's a good thing. In vanilla, it's too advantageous to make as many small cities as possible, which the AI would never do; in the short term you might be limited, but eventually you'll be unstoppable. Giving the player a good reason not to settle as often as possible is one of the cornerstones of the Balance mod.)
The thing is, growth is a two-edged sword that benefits you in the long run; sure, you'll basically need the equivalent of an extra luxury or more just to stay even on happiness in the short term, but your cities will be much more productive, produce more research and trade route gold, and so on. Even if it drops you into negative happiness, you won't have enough cities to go THAT far down; build/buy a Colosseum or two, or trade for a new luxury, or bribe a new city-state, and you're set.
There's just no way that I'd give the Hanging Gardens enough happiness to balance out the unhappiness caused by its growth. If you had 5 cities, then that's -6 Happiness, which'd take a Wonder in its own right to negate. Now, what I can do is what I did to the Human Genome Project (which has the same basic effect): add a token +1 or +2 Happiness to your empire. Just enough to make up for that extra 0.2, but not enough to break even. (The HGP will come when you have a dozen or more cities, so obviously +1 population can really hurt your happiness there despite the +1 happiness it also gives. And then consider the Cloning Vats, which give +2 population and no extra happiness...)
The other advantage of doing this is something I've run into in several games now: there are a handful of Wonders, mostly early ones, that have no "persistent" effect. Capturing a city that has the Hanging Gardens in it drastically increases your score (which makes the AIs hate you more), but you get almost nothing out of it. So adding even a token amount of happiness would help with this.
This sort of unhappiness issue also encourages players who want to follow the growth-at-all-costs strategy (whether the Hanging Gardens or just normal growth) to go out of their way for policies and wonders that reduce unhappiness (the Piety branch, the Forbidden Palace) instead of going for the Rationalism-style boosts that'd stack with the increased research from larger cities. So it's self-regulating; if your empire's growth is causing unhappiness to rise too quickly, start shifting to things like Production Focus to slow down your cities' growth and/or put more emphasis on gaining happiness.
The alternative is to change the Hanging Garden's effect to something less problematic, like "all cities get +10% to food production". I've done this for the Longevity Vaccine; you get increased growth, without the instantaneous jump in unhappiness. But I'd prefer not to alter the existing wonders.
