Can I just pay for Internet on iPhone 4?

I have an iPhone 4 and my bill evey month is too high for me pay for anymore. So I bought an other phone to use just to talk on. But I still want my iPhone 4 to use for Facebook and stuff. Can I pay a low price every month to just have Wi-Fi on it every month.

Why is it high?
Do you use over 2 gigs while you're in 3G
It really shouldn't be that high.
Only if you don't have unlimited text and calling?

I think if you're gonna get another just for text and calling, just buy an iPod touch or iPad

Well after all, it depends on your internet plan, the speed of the internet etc.

You don't have to pay a carrier for Wi-Fi usage. They have nothing to do with that.