A search might have helped you, the Thai domestic rates have been discussed here about 678 times... Anyway, TG fares are set - no discounts.
BKKHKT Y THB 2675
BKKHKT C THB 3475
Web booking gives 10% discount at the moment when you choose Royal E-Ticket. PG/9R might be a little cheaper but not worth it IMHO. Buy C on a 747 and ask for a seat in the F cabin. TG 921/220 combo gives you meals both ways, TG 920 doesn't have a meal.