Think about the different pieces of the pie.
1. Amount per hour
2. Amount per project
3. Location work needs to be completed
4. Deposit
5. Payment terms
6. Time to finish the project
7. Specs and reporting

All of these can help you negotiate their rate. The important part is to ask up front what they expect for each one of these, then ask of there's a part, if made more interesting can help reduce the rate (ex: 50% up front, or Net 0 vs. Net 30, etc)

You just want to ensure they do make some money so their invested in your project - you don't want someone working for you who doesn't want to be there.