I seldom build marketplaces for increased tax revenue (increased luxuries are another story), and even more seldom do I build banks. The reasons are simple. A marketplace is worth 80 gold up front if you sell it. You have to produce 4 gold before it gives you any benefit at all (2 extra taxes, less 1 for maintenace). That's a return of 1.25% per turn; hardly a good return (the player Peaster has argued that Civilization gives an interest rate of about 5-7%, depending on if you are playing for spaceship or early conquest). Also, I usually buy my improvements, so the real cost of a marketplace is upwards of 160 gold; essentially cut the interest rate in half. Banks are even worse, costing 3 gold to maintain, and are more expensive to build. Moreover, a good caravan delivery can get you hundreds of gold (and an equal amount of science) for 50 shields and the effort of transporting it to a good destination; far better than building marketplaces and banks, not to mention the permanent trade routes that you also get.