Step 1: Go to Target. They're selling T-mobile prepaid cards for 10% off. Get the $100 card for $90. It's important to get the $100 card, because T-mobile has a deal where if you buy a $100 card, you get a 15% minute bonus and your minutes don't expire for a year (as opposed to 45 days with cheaper cards).
Step 2: On your way out of Target, pick up this week's sale flyer. In it is an ad for a Nokia 6010 phone for $35.
Step 3: Take the flyer to Staples and buy the 6010 phone there. Staples is selling it for $80 with a $30 rebate, but you'll be able to price-match the phone with the Target flyer, so you'll get the phone for $35. Don't just print out the flyer from target.com, as the Staples folks will cross their eyes at you. If you've got a bona-fide flyer, they'll take it.
Step 4: Activate the phone and apply the phone card.
You've now got a quite-nice color phone and 1150 minutes that you can spend over the course of a year for $90 (minutes) + $35 (phone) - $30 (rebate) + tax.
If you just want a cheap unlocked GSM phone, you can just get the phone and go here to unlock the phone so that it'll work with any carrier. Tell the form that you've got a Nokia 3510 with ASIC 6 circuitry, and it'll give you a code that'll unlock the phone. It did for me.
Oh, and if you want a car-charger for your new phone, DO NOT buy it from a cell-phone store. They'll charge you ten bucks for one. Nokia phones all use the same charger, and it's a good bet that your local 99-cent store sells 'em. The one near my house had plenty.