Well, there's two apporaches to this. Whether or not your being selfish is debatable, I perfer not to bring the reply THAT far into it, lol
1. Get the PS3, enjoy it, and tell your mom you wanted to buy it on your own money bacause you didn't want her to spend that much money on you..or
2. Wait it out a month, and have your mom get it for you, and keep your money to yourself. I recommend that if you go with this options, extremely thank your mother, and show your appreciation, PS3's cost ALOT.