I know this sounds cheeky but it's called research, go around local salons and go on mobile stylists websites and look for te price lists, i ignored the stereotype that mobile stylists should be cheaper ad went mid range salon price's due to myself being convenience for the client by going to them, I'm no less qualified to do their service than a salon so why should I be cheaper?
Don't sell yourself down! Like has been said keep prices higher and if needs be lower them or run promotions.
When I first qualified a friend of my mum had her cut and blowdry for £5, she would sometimes buy a box colour for me to put on too and still wouldn't pay more. When I decided I was going mobile I let her know my prices would be increasing and that no one was getting discounts other than my promo's (that's a lie as my mum is free as does her best friend for the huge amount of help she has given us over the last few years with family members dying she kept my mum sane, so all I ask is they cover the cost of te tube when I need to buy it.)
You have to slightly remove yourself from being a business owner strikes as see yourself as an employee of that business, as just an employee you don't have the authority's lower the price for anyone. That's how I do it and just tell clients the price is set and not for negotiation.
Remember being mobile includes lots of wasted time, traveling,
setting up, waiting for colours to take... So make sure you cover these costs also, otherwise you may actually lose clients being so cheap, how can she charge so little? Maybe she's no good? Is some things that will go through people's minds. And although it's expensive to begin with , get a quality set of product, quality tints and possibly even replace what's in your college kit with better quality as these are really just beginners tools, it all shows through as you charging accordingly. Xoxo