Pretty much all that is necessary for an "economy" to develop is some form of scarcity, the ability to trade reasonably freely, and the ability of the traders to set prices. If you've got that, you'll get a market of one sort or another, and it's not hard to even accidentally end up meeting one of the conditions when you didn't mean to. Charging flat fees or usage-based subscription fees is not enough to guarantee that an economy will still not emerge... MMORPGs have those things and economies, too.

(There's not much guarantee about what sort of economy you'll get... it's easy to get one that pathological in some sense, either psychological or mathematical. But you'll get something.)