It used to be €30 ($41, according to the good sis Google) for my iPhone 3G S 2 years contract, but it ended a few months ago. So since I've now payed my phone off, it has changed to €15 ($20).
My bill is like $86/month with AT&T. I refuse to go higher than that, although my bill once was over $100/month. Sacrifices had to be made because that was killing me.
If I topped up £10 one month I'd get unlimited texts and internet access for the whole of the next month.