Take the thousands part of what you would consider to be an acceptable annual salary and double or triple it; this becomes your hourly rate. I'd start with tripling it (or more), and going down from there. You're better off to come in with a high quote and work your way down, because (a) raising your rate is a lot harder than lowering it; and (b) the client will feel happier when they've worked you down a bit because they feel like they're getting a deal.
So, $25/hr works out to you being happy making $12k/year.
Start off by telling them that you usually quote $100/hr (or whatever you feel comfortable with), and if they balk at that, follow it up with something like, "But since I really like working for you and want your business, I'll drop down 10% right off the bat."
Don't feel bad like you're overcharging them -- as long as you do good work for what you get paid, both you and your customer will benefit. It's tough to walk into a conference room and ask for what you feel is a shocking amount of money, but this is because most geeks (myself included!) have a habit to think we're worth less than we really are.
And, who knows, you might actually get the higher rate that you quote.