I think my decision to become a freelance Drupal developer and themer about one month ago is very timely decision. There are two kinds of rates that I charge to my clients, hourly rate and fixed rate.
For hourly rate, I charged when a client requires only small things like adding or fixing the CSS, create a simple form and others small things. My current hourly rates is $??, contact me for info regarding this :). For fixed rate, I switch to this kind of rate type when my clients need a big projects, yes like creating a custom theme, creating a site etc.
Currently I still restrict my availability every day because there was one project (custom Drupal theme) which is still under developments, almost finished just need some touches. Currently I only received one to two hours per day, my time is very limited now :)
For custom themes, I only accept a theme that the budget is $1000+, just contact me first for my availability and I will set how many days I could start your project.