If you are looking for a basic cell phone, I think Bangkok will probably run a bit cheaper. You will find the best prices on used phones - there's a lot of swapping of barely-used phones for newer models with more complex features. I'm not sure that you really need to seek out Panthip or MBK - any shopping mall in the city has a cellphone center with several vendors and all sorts of prices. Nokia seems to be the most popular brand, but all the major manufacturers are well-represented. As for SIM cards, the best deal I found was for 40-baht at a temple fair.
