- your monthly expenses
- how much of your income comes from the work you're pricing (do you have other sources of income?)
- how much of such work will you be getting this month (and how much you'll be willing and realistically able to handle)
- how long do you estimate the work to take to finish (you really should have a good idea before you start taking commissions)
- what are other artists of similar skill level charging
- are there any additional expenses to factor in (software, materials, documentation for tax purposes etc.)
This should give you a pretty good idea where you stand.