You're not only paying for quality dyes when you go to the salon, but experienced hands applying it. Cosmetology students will often do your hair for very little money, if it's not free - they have to log in a certain amount of time working with hair as part of their training. No need to buy cheap dyes online.
I agree 100% with Ceebs. I have died my hair countless times and just about every shade possible. I REALLY regretted the times I did it myself. It is really worth it to go to the salon. As Ceebs said you can usually get a great job done by a student. My friend used to get hers done all the time at a student beauty salon and it always looked great!