The only airline serving REP (from BKK) is PG and with their monopoly comes a high price indeed. PG has a Discovery Airpass which can be a good deal for international travel however it's of very limited use (if any) to REP and you can can't get to DPS with them. *A carriers fly to DPS (TG, SQ...) but not to REP.

Air Asia flies to DPS from BKK. They can be low cost but only if purcahsing tix far in advance. Close in they can be more expensive than full cost carriers.