I suspect that's very hard to do, for the reasons you suggest. When I did freelance work, I always did the initial site as a fixed cost bid against the specs, and follow-up work was hourly. Unless your client knows you well, it's hard to imagine them accepting a "I will just work until I'm done and charge you by the hour" kind of project, it's just too open-ended, even if they trust you're not cheating them. That's true in most fields, not just web sites. Imagine a plumber coming to fix your pipes telling you "I have no idea how long it will take, just pay me $75/hr until I say I'm done..."