Don't worry about getting an expensive ring... That's the mistake my brother always makes, he always has to buy the most expensive thing, and really it isn't worth it. They won't love you any more or less no matter what you buy them unless they're in it for the money. They know you're in uni and don't have the cash to spend to be able to buy something decent.
If anything, promise her a really nice ring for when you have a steady job ^_^ and after you're married of course
