I have bought AIS 12Call and DTAC SIMS in Bangkok. I paid 50 Baht for the card and added 200 Baht of call time. I used mostly to receive calls from home. I just bought one at 7/11 at BKK after aexiting through customs.

I bought a Cellcrad SIM in Battambang Cambodia for $2 and aded a dollar of talk time. I sent an SMS to my wife with the number and got a call back within minutes. Once again, primary requirement is to stay in touch with my wife, I may use it to book a hotel.
The website says the card is preloaded with $4 but it still has to be topped up to activate. However when I added a dollar, it showed only a dollar of credit. I would end up not using most of it any way.