If you're playing a map with layout/opponents that lends itself well to espionage, it can be massive.
The most important aspect is correctly identify strong espionage targets - somebody who techs well but is not a serious threat, keeps borders open at bad attitudes and is close (mansa/capac/lizzy rock for this) you can steal every tech with ease for the second half of the game.
What's more - you can keep up in tech easily with nothing more than prioritizing the espionage buildings in your cities (jail/IA/SB mainly.)
If you prepare target enemy cities for stealing techs, the results can be quite absurd. Here's some things to do:
- Pick a close target (no more than 10-20% distance penalty)
- Have open borders (much less chance of spy detection, 20% cost reduction)
- Spread your culture in the city (easily get 20-25% cost reduction)
- If you've gone espionage heavy, you'll probably have 25-40% reduction from total espionage spending ratio.
- Get a holy city, get your holy city religion in the target city, make sure he's not running your religion or he's in FR - 40% discount.
- Wait the 5 turns for 50% discount, much easier to do with open borders.
- (V Important!) use spies to destroy the security bureau in the city before tech-stealing. This makes all your other spies in the city have much less chance of being caught while idle, higher chance of mission success and reduces the mission cost by 33%.
So let's do the maths for a tech. Take a cheap tech, like fusion(!), at 20K beakers on Epic/Large. Note the following is what I have achieved several times in real games, it's not just theoretical number crunching.
Base mission cost = 20K * 1.5 = 30K
* 1.1 (distance) = 33K
(no *1.5 for security bureau, because you destroyed it right?)
* 0.6 (holy city/relgion)
= 19.8K
* 0.8 (open borders)
= 15.85K
* 0.65 (total espionage spending)
= 10.6K
* 0.75 (about 20 spread culture missions normally, but they're pretty cheap so the total cost to get the discount is made back by stealing about 1-2 techs.)
= 8K
* 0.5 (stationary spy)
= 4K
so .. I'm getting an 80% discount on techs using prepared espionage cities. Prepared cities in two well-chosen target civs is usually enough. Once you've prioritized the espionage buildings and spent a bit of time with a high espionage slider, you can drop down to 0% and still maintain high total EP ratios. Obviously the above is the optimum case, and I'd rarely get *every* one of those bonuses, although it has happened. If you drop the culture spread and holy city, you're still getting a 62% discount, or paying a bit over 1/3 price.
With just 8 cities with the EP buildings you'll get 352 free EP points. Due to each city having +100% espionage, running something like 20% slider should increase this value to around 700. That means you're getting the above fusion tech in about 6 turns, or an equivalent beaker rate of about 3500K per turn, with 20% slider. With 8 cities that's an equivalent beaker rate of 440 per city - with 8 cities you will seriously struggle to ever tech that fast using beakers, even with 100% science slider, full cottaged and low production.
With the great majority of your "research" coming from a few buildings in each city, this frees up the rest of your empire to concentrate on gold (paying the bills and upgrading troops), rush buying, production workshops, etc. Of course you'll never have a massive tech lead (you can achieve a lead by sheer breadth of techs if you choose good targets) but then again this strategy works best on immortal+ where you're unlikely to have a good tech lead anyway. It's kind of a waste of time on anything up to and (imho) including monarch - you're just stunting yourself by "only" keeping up with the AI on these levels.
Once you get it up and running well, it kinda feels like having the internet for half the game. Very powerful. A common strategy of mine on Immortal is:
-cottage heavy initial cities -> beeline rifles -> drafting war -> communism/democracy -> state property/caste -> workshop all commerce tiles less than village -> EP buildings everywhere to keep up in tech -> full military -> domination.
works great.