The economy system would need to be a massive addition, probably for later. Since at the moment you can't just dock up to the local Arcadia and buy a nice new set of Harps or a fuel pod for C7050, the money thing would probably be restricted to scripted encounters for now- points where you, say, talk to another pilot, and a y/n keypress determines the transaction. If you agree, a certain value is deducted from your cash, and you get information/access to a new weapon for the next mission.
In the end, that's probably as far as it will go. FreeSpace isn't particularly amenable to a trading game setup, and while it's technically possible given the current sexps and a budget system, it'd be a lot more work to set up some freeform universe than a simple linear campaign. Think FREDding fifty to two hundred missions, about half of which were in the same places as other ones, just with new friendlies/enemies and mission parameters. And it would STILL not be a fraction as good at it as a game that was made for this sort of thing.