Not sure where you are looking, but TG fares on their website seem to be more in the US$260 range for round-trip on this route. Still not a great price, and their website is notorious for not having all fares (esp the lower ones) loaded into the system. Also, I believe TG just raised their fares on domestic flights--the Thai Subforum under "Miles and Points" has more on this.

If you haven't already, you might check Bangkok Airways (PG), also a good little full service regional and domestic carrier. You won't earn points on your normal FF programs, though. Often considered a relatively expensive airline, sometimes they can surprise you with decent fares. I was just looking at their website for flights BKK-HKT around mid-May, and was finding web fares for round-trip total of $130 (Bht 4300-ish). With that difference, I'd fly them over Air Asia anyday, particularly as they tend to stick to schedules, and have no nickel-and-diming hassles of LCC's. Checked baggage allowance is standard mainline carrier 20 kg per person. www.bangkokair.com

If you are going to start out in Bangkok for a few days, you can also just wait until you arrive, and pop in to any travel agent who can get you a discounted e-ticket.