I asked at the airport testing service, RT-PCR tests are available for 1200 bahts, results are given the same day if test is done before 11 am, otherwise next day. Antigen tests cost 250 bahts with medical certificate, 150 bahts without, 15 minutes waiting time
Suvarnabhumi airport, floor 1, between entrances 7 and 8, outside, 1200 bahts, result in 24 hours. Not in Silom/Sathorn area but it's the cheapest and getting there is only 35 bahts with the airport link from Makkasan