A1 stops at Mo Chit and that's also the final stop
A2 also stops at Mo Chit and then on to Victory Monument
A3 goes to Lumpini Park but does stop close to CentralWorld
A4 goes to KaoSan Road, I don't think it has any drop-offs near a BTS or MRT hub.
I know for Mo Chit it's 30 baht but maybe 45 if you're going further. The buses use the expressway from the airport to blow past traffic, on average 15 minutes to Chatuchak Park (Mo Chit).
Over the past few years they have also upgraded the rolling stock from the Orange A/C busses to newer blue buses that actually have disabled access and seating too.
As long as you're travelling pretty light, it's super convenient.