There's no such thing as a "student developer". If they pay you, you're a pro. There are only good and bad pros. Get into the habit, right now, of thinking and acting like a consultant running your own business. If they're paying you money, they expect you to behave like a pro.
Setting a bill rate is the third most important skill a consultant can have. (#1 is ability to do work, #2 is ability to promote yourself). You have four goals:
1. To work steadily enough to make a good living, but not get overwhelmed by assignments.
2. To build a base of clients you enjoy working for-- who do not cheat you or drive you crazy.
3. To do interesting work that you enjoy, which also builds your skills.
4. To make the world a better place or become famous.
You use those factors to modify your standard bill rate. If you want the project, you reduce it. If you don't want it, you bump the rate.
Since I don't know WHERE you are (how much competition you have, or how much local businesses expect to pay), it's silly for me to give you a figure. SF or NYC means you charge one rate; Nashville or Pittsburgh means another.
A simple way to get a feel on the local market is to call up the 2-3 biggest body shops (technical temp agencies) in the area, say you're a project manager for a big local firm that wishes to remain confidential (so they're not overwhelmed with unsolicited offers) and are looking for 2-3 iOS developers-- a senior guy, an entry-level and a decent worker. Ask what the current rates are.
In less than an hour, you'll have enough information to give you local market rate-- low to high end. (This is not "dishonesty"-- it is "growth hacking" for your company.)
Your line when asked will be "It depends on the project. My rates run between [entry-level] to [high-end], but it depends on what the project is and how much time you want. If you're looking for a ballpark, let's say [average]."
Nine times out of 10, the client already has a budget and just wants some idea of whether you fit in it.
Some people will say "Great!"-- which means they're willing to pay you the midpoint you gave them.
Some will say "Well I can only pay you $$$$" (a dollar amount, not a rate). Now you know the budget and can immediately say "Tell me about the project" (so you can cost it and see if you can do it for a decent rate).
Big places will usually tell you what their standard rate is.
And when someone immediately begins haggling about money, run. You don't want them.
Once you have that, you use the following rules:
1. NEVER ever EVER do fixed-price jobs. The minute you do rework, you lose. Find out what the project is and figure out, based on your time estimate, if you can get a reasonable rate for that. If so take it. Otherwise, pass.
2. Discount for projects that require large blocks of time. 8-hour projects are hard to schedule and they often require as much pre-sale time and hand-holding as 40 or 500-hour projects. That's time you're not billing or learning, so give then no breaks.
3. Discount if the name of the company sounds like someone you'd want as a client or in a portfolio. (Not too much-- big places expect to pay market rate.)
4. Discount if the task sounds fun or like a good learning experience (if it's a learning experience, you charge less because you assume you'll blow stuff).
5. If you've you've heard bad things about the company or you don't really want the work, sock em.
Some of the other advice here is pretty good, but these are the basics that will take you through a career.