I do a fair bit of this sort of contract work. I normally either charge $100/hr or give a fixed price. For project work I normally charge a fixed price, but I make sure I get a reasonably detailed spec before doing this (and I insist on not doing any more work without further pay, unless it's very minor stuff).

Note that I've got 20+ years experience, and ALL of my contract work comes through my website (collaborative whiteboard/conferencing software) - normally it is customizing the software to the customer's requirements.

It's very difficult just finding this sort of contract work unless you have some way to funnel in new customers. In your case I'd recommend maybe just taking on a full-time on-site contract job.