With Alltel U prepaid you get the choice of 2 out of the 4: unlimited SMS, unlimited N/W minute, unlimited calls to one number, or unlimited M2M; all for $.75/day.
Pay-Per-Day: U Prepaid Wireless Service from Alltel
They have some cheap flip phones.
If you need less then that, check out
Welcome to TúYo.com
Tuyo Mobile is $.02 in and $.05 out for SMS. They use tmobile's native network. They are
GSM so you can put then
SIM in any tmobile "branded" or unlocked
GSM phone with 850/1900 frequencies. So phone choice is yours.