Hi, its tough i know! me & my boyfriend are both 18 and been going out for over a year now, but its hard thinking of something which is romantic/memorable without costing too much..
maybe for a little thing you could get a picture of both of you and put it in a frame for him to have at his house?
For my boyfriend this year ive got him an ipod shuffle and put all his favourite songs on it, though that will probably be a bit much for your budget so why not get a cheaper mp3 player & put his favourite songs on it.. although not a good idea if he already has one.
Or what about getting a dog tag & having it engraved.. i dont think it would be too expensive.
im stuck for idea's now!
hope this might help.