Rates are negotiated based on three things:
1. the size of your painting
2. the time it will take me to complete your painting (depending on style and subject matter)
3. the materials I use
In general, I charge more for portrait painting because of the amount of time and materials I need to complete quality portraits. Landscapes and abstract paintings, on the other hand, will not take as much time or materials, therefore I do not charge as much for these.