Yes, the whole player-driven economy thing is really hard to do, and probably near-impossible with a free-to-play model. I'm not really concerned with pricing models here, just the gameplay as they described it.
On a different note, I'm tired of stealth games that get less interesting the more you try to challenge yourself with them. It seems like almost all of them front-load their mechanics into a combat system, and shooting your way through a level (even if you're doing so in a stealthy way, not just running and gunning) is inevitably the easiest way to play. Then they convey the message that the greater challenge in the game is a "no kill run". And it almost always is a greater challenge, but it also turns out to be less fun than killing everybody, because it's a "no fun gameplay mechanics allowed run".
Stealth games need to start pushing in the same direction as character action games. Those get more rewarding the better you get at them, because the deeper mechanics are hidden behind a more user-friendly "surface game". The game starts out fun but simplistic, and the player can more or less button mash through most of it on a lower difficulty level. Higher difficulty levels and a grading system then encourage the player to replay the same areas and improve, perfecting their understanding of the mechanics.
I don't know how you would do that with stealth gameplay mechanics, but moving away from a binary win/lose state (a stealth equivalent of a health bar?) might be a possibility.