In my limited experience ebay is the cheapest place to buy mobile phones.
I noticed a 630 going for $190 USD but you can search completed auctions to get a feel for pricing.

If you do go to Bankok, go to the MBK (Mah Boon Krong) shopping center, go the 4th floor and you will find hundreds of mobile shops/counters. Prices on most phones here are ~ 0 to 40% higher than ebay, on both new and used phones.