How much is it for an iphone 4s or an iphone 4?

So i'm planning on buying an iPhone 4 or 4s I'm not sure if their both the same thing but i'm just 12 so I don't know. I want to know how much it costs monthly, I also need some information from those of you who had had this iPhone 4/s. I don't have much money so don't tell me to get an iPhone 5c or whatever. Right now I have about 300 dollars in savings. (I work for money) I get paid 40 dollars a month so I think I probably have enough for iPhone 4. Please ANSWER.
p.s. I have T-Mobile.
Thank you!

Added (1). my poppy will get mad if I don't buy a phone in 3 days.

They should be around $200

Tmobile doesn't appear to sell the 4 or 4s. 4 is probably gone. They don't make it anymore. 4s will be gone by September.
http://support.t-mobile.com/...s/DOC-4771
Unless your local store has one refurbished in the back.

You'll have to buy an unlocked 4s from an old carrier like ATT to use there. Be careful what you are doing when buying too.

Should be doable for $300 or so. Best luck.

The iPhone 4 is discontinued. The iPhone 4s is free on Verizon and it will cost about $35-$40 a month.