You should choose the prepaid plan that gives you the choice of Pay As You Go, Pay By the Day, or Sidekick. You can switch among them at will. Pay As You Go and Sidekick both allow you Internet access. I switch to Pay By the Day on days I am at home using Wireless. It costs nothing when you don't use it.

Sidekick gives you unlimited Internet access for $1 a day, but it costs $1 every day, whether you use it or not. So I only switch to Sidekick on days when I plan to use the Internet via Tmobile.

Whatever you do, once you are on this prepaid plan, the costs are low.

BUT, there is a catch. See the following thread:

http://talk.maemo.org/showthread.php?t=37887&page=8

in particular message #72, the numbered one by me. You need to use either a proxy or Tor. Tor is easier.

All this looks intimidating, but it is cheap and interesting and can be fun.