How much money should I spend at this anime convention?

Im going to be going to Tekkoshocon (this is my first convention) on march 31st to april 3rd and I was going to take $325 (with the badge already covered, other money for food, and im not paying for anything else(im 15) but the more I think about the stuff im going to buy the more I realize that there is no point to getting a lot of things that are going to be unnecessary and expensive (at least until im older, have more money, own place etc. ) and with the Nintendo 3DS I was thinking of dropping 250 on that and then having 75 dollars to spend at the convention. All im thinking of buying is wall scrolls and posters, either that or I could just take the 350 with the 3DS in mind, what would you do in my situation? While buying a blade from bleach, a zero mask, an evangelion or gundam, a death note or random figure would be cool, in the long run it would be better to get a 3DS, and then on the wall scroll side of things, stuff like hellsing and gundam will look badasss on my wall, but I can't exactly put up cc from code geass or lucy from elfen lied or else my parents would think im a ******** weirdo (im going with a friend) anything im overlooking that I could buy that is inexpensive at a con and has some use? I love figures and blades but a new DS kind've takes priority since its a brand new system I will be looking into for at least 9000 hours of my life, I got 350 for Christmas, and once this money is gone it's GONE, I don't have a job, I don't get an allowance, I love anime to death but I can't exactly throw money around like its nothing since, once again, 15, no job, won't have this kind of money again for a looooonnnnnnnngggggggg time, what should I do?