Please pardon my noob question, but I've been playing since Galciv1, and I just don't see the point of economic planets. Do the markets stack multiplicatively? Or are you just aiming for a high score, what? By the time you're done pouring all that extra money back into your planet to build more stock markets, the game's over.
In a tiny map and/or with few opponents, sure.
I'm not sure what you mean by "do the markets stack multiplicatively". Each market adds 25% to the planet's base income, so it's more like simple interest than compound interest. In essence, this means the first four markets are worth "more" than the next four, as it basically doubles your (planetary) income. Additionally, the markets are planetary-only...they do not have a civ-wide effect, but building even one or two on each planet doesn't take much time (at last check, stock markets cost 120 industry to build, or 1158 to rush buy-making the former the more attractive option) and is a fairly large income increase by percentage.
However, one can think of the average of the number of stock markets on a planet as being a civ-wide bonus if the planets are equal in population (and therefore tax rate, as you can't set tax rates individually per planet). Further, this is from a base of 100 as well, so it multiplies by your civilization's economic bonus, rather than adding to it (note: for those of you to whom it isn't obvious, this is better).
When you consider that the two outliers here will be your homeworld(s) (for instance if you take over another civ's homeworld, even if it's a minor race) and planets too small to reach suitable "max" pop (basically sub class eight), which even in a tiny map are relatively small groups, the average population is very close to your mode. Not to mention the fact that as tax income is a function of the square root of the population, your homeworlds actually are not making that much more (except in TA, where they also have a 10% economic bonus on the civ capital) than your average world and for our purposes can be safely figured as a normal world.
Essentially, an average of one stock market per planet is 25% more income than zero, two is 20% more than one, three is 16.67% more than two, four is 14.28% more than three, five is 12.5% more than four, and so on. Each successive stock market provides less of a percentage benefit to your overall economy from the one previous, so on smaller maps it may only be justifiable to build one or three, but it's still worthwhile.
Then again, if you don't have economic problems, you can safely ignore them.
However, as I'm sure a number of players will tell you, if you aren't having economic problems, you're not expanding fast enough. Which isn't to say that having economic problems is a good thing, but it's a rule that I personally live by (in GCII).
(Note: Obviously the above numbers ignore the fact that your planets aren't always at their max pop, more so when you've just colonized them. It's safe to say if your games don't last long enough for all of your planets to grow to max pop, then you can ignore this post-and stock markets, for that matter.)