Dude, it's up to you. What did it say on the prom card/invite/whatever? Usually they state the dress code. If it's formal, a blazer and khakis are not formal enough, no matter how nice they might look together. But a simple google search of the word "prom" or "prom suit" will get you similar images:
So, if you don't want to rent, just buy the suit if you can.