The cheapest prepaid wireless I have found is Boost Mobile. This runs over the Sprint / Nextel network. They only require you to purchase $20 every 90 days, which is the least of any prepaid service I could find. This works out to $6.67 per month and any dollars roll over indefinitely.
T-Mobile is more expensive for the first year (you have to spend $8.33/mo) however you can then spend as little as $10 per year ($0.83/mo) to keep it active.
Oh.. and no taxes or anything on the minutes like the other companies charge. I use cheapphonecards.com with coupon code cheap (this code does not reference me, I found it on google) and get the $100 card for like $92.
