Ask HN: What to pick for an hourly freelance rate?

I'm a student iOS developer and I've just completed paid freelance work (paid by the project). It was ridiculously cheap as I needed a portfolio, but my client is now asking for my hourly rate to continue. I'm not sure what to give them, or any future clients.

Here are the details: I have a portfolio of two previous freelance applications (price essentially free to boost portfolio), an app on the app store (Firework Flare) and have had an internship at an app agency (worked on apps for Unilever).

What would you expect me to charge for part-time freelance work?

Thanks HN!