What is the usual cab fare from Suvabhumi airport to downtown Bangkok?

May I suggest taking the train into central Bangkok then a taxi to hotel. It's on bottom level of airport. Quick, cheap and safe. Room for luggage. About $2 or 70 baht (don't need express which is more).

Best driver will turn on meter, warn you about toll and airport extra fees, and deliver you to destination in midtown BKK for max of 400THB. Waiting time in taxi is today poorly recompensed, good for customer.

Bad driver will say 500THB and no meter, flat rate. If you give hotel destination, this is more likely to happen; Instead choose an address or big building NEAR your hotel, and upon arrival, just say :go to that hotel over there."

Smile and laugh if taxi asks for extreme fees; he is just trying to make a living like the CEO of a big corporation... just another capitalist.

Whatever it says on the meter +B50 airport surcharge + toll fees if applicable. So B300-350 depending on where exactly "downtown". Absolute Max would be 400.