I think you should evaluate your financial habits, work out what you spend your money on, and where you get it (ie: where money comes in and goes out). Then you can go from there.
Back when I was 12 I never really spent alot of money (well.. there really wasn't much for me to buy back then) so I guess I was a pretty good saver. Would you consider yourself a good saver? If not, it might be an idea to learn how to save
As far as acquiring the actual game goes... If brand new is too expensive for you, try browsing around the Sunday markets (over the years I have found some absolute gems there), Pawn Shops (once again, you never know what you'll find there, and at killer prices), and last but not least the paper.
I don't know where you live but assuming you're paying about $100 I'd say it's a pretty accurate guess to say you live in Australia or NZ due to currency scaling (if you're paying that much in USD you're getting ripped off my friend).
I live over in WA, we have a great 2nd hand merch catalogue called The Quokka, and I've found some absolute crackers over the years. Cash Converters or whatever pawn shops you have (Cash City perhaps?) or Trash n Treasures (they're on every sunday at numerous places around the metropolis).
Don't forget dude, unwanted gifts tend can be flogged off pretty cheap, as alot of the time the seller tends not to have a full understanding of the retail value of what they sell (I have sought many-a-bargain from a seller's ignorance - That must sound hell scungy! )
Hope this helped you in some way!