Currency for me is usually a 'late' AA-tech (but see Justanick's point re. Construction above: possibly I'm also leaving Currency too late), Markets are expensive (100s), and they cost maintenance (1gpt), but aside from the Lux-boost, they boost only the TAX%-slice of CPT-income; if I'm running high-SCI% and low-TAX%, they don't give much if any economic benefit (but again, see Justanick's point about minimising SCI% after going to Republic, and cash-rushing improvements). That's why I generally only use Markets as Lux-happiness magnifiers -- but that only applies if I have access to >3 Luxes, which also doesn't usually happen in the first 100-150T (unless I've rolled a Pan-Map, or a 60% Continents). And even once I have my 3-4 Luxes, a Market happy-boost won't be needed in newly acquired towns until around Pop6-7, so that's when I'll build it.
Conversely, at higher levels (and on Large+ Maps), where buying techs may well be faster than researching them, Markets + Banks + TAX%=100%, rakes in the coin, so may be worth building earlier...
Just to be clear: I rather advise against building market places for the purpose of cash-rushing improvements. Courthouses are usually more valueable than market places and aqueducts are more valueable than courthouses. Once those 2 buildings are up and running your regular production should grow so much that it will suffice for building what you need. Cashrushing is meant to be the exception, using it for research should be the standard operating procedure.