If the bag is small enough for carry-on, you should be able to arrange for delivery of the bag to you in the transit lounge....but the fee for this service might be higher than 500 Baht. They cannot check the bag for you (for security reasons), so if it is checked baggage you have no choice but to clear customs and go landside to get your bag.

Not sure if BKK has a departure tax exemption for same-day arrival and departure, but many airports do so it might be worth checking as you probably need to get a special form stamped upon arrival.