how short term are you looking for.? . there are many places around rama 9 mrt. . there is a place that does month to month in ratchada rama 9 for around 8K. . my friend stayed there it wasnt great but it was month to month
or there are a ton of really nice apartments in this area that will do 6 month leases for 8-12K depending on view and size. . pools, gyms, etc. . .
let me know more details and ill give u some info. .