Also, rates depend on the work required by the client. If you are purely doing some frontend/interface work, it is reasonable to adjust that rate differently from backend work.

Another note is that rates should fluctuate depending on timelines from the client, their budget and requirements. A healthy balance of determining all those factors and your ability to estimate your abilities to complete work can result in a wider range of price ranges - either fixed/hourly/weekly/monthly, etc.

Most of this can only be figured out through lots of experience.