I'm not being mean, I am trying to help but... $500 will get you a great toy keyboard from Wal-Mart. Anything that is a real instrument is going to set you back some real change.
If you are near a pro-style shop ask for help, if they have a keyboard already, the smart choice maybe to purchase an expansion module or a rack mounted tone generator for them. Otherwise, Aeolius is on the right track. Music software (the good stuff mind you) would be a great gift.
Hope this helps. If you need further specific help - email me, a real answer might take some time to get a good answer for you.