If you have not already, you may want to join the Dr S yahoo group, that's an excellent forum to find others headed that way at the same time as yourself.
Personally, I haven't heard of anyone (successfully) haggling over the price of SRS with Dr Suporn's office, but who knows

. I don't think it's any huge secret or floating schedule to what Dr S charges for SRS. Everywhere that I've seen it mentioned, it's been the same amount of baht for the last few years.
Take Care, Good Luck!
Melan