If you want only a dual band, no frills phone, Bkk shopping malls is your best bet. Used or barely used phones can be had for well under $100.
I recommend 1-2 Call prepaid sim card .
At the "Mall" in Bangkapi area (which is out of town - but you can find comparable in town), downstairs in department store, you can get 1-2 Call, pick your own number and I think about B120 with B100 minutes. In same Mall, I think level 4, there are tons of mobile phone kiosks.