Piece of advice, you're a senior: you won't see her again 4 months from now.

That ring, on the other hand, is $875. Now I have no idea what the hell an 18 year old is doing with a ring that expensive, but I do know that giving anything even remotely close to that ring in monetary value for Valentine's Day is patently retarded. Go get her tickets to a movie or something. Write her a card and promise to take her to an upcoming concert so you can scrounge something in the meantime, but don't do anything that stupid.