People who have t-mobile and know a lot about it..please help?

[kerrrry ?]

okay so my birthday is tomorrow and i want a new phone. my mom said the androids and blackberries were to expensive...not the phones but how much it is a month (30-60 dollars a month) so she told me to pick out a different phone but i don't want to. so i was wondering if i bought one of those phones off ebay and put my sim card in it, will it make me pay the same price every month that i would have to pay if i bought it from tmobile? because one time my cousin gave me his android phone (motorola cliq) and i put my sim card in it and it didn't make me pay 30 dollars a month and it had it listed that i still had the gravity 2 not the motorola cliq. please help. 10 points best answer