I take the taxi all the time. They generally will go off meter for 400 Baht. From the airport it can be anywhere from 300 Baht to 400 Baht depending on traffic. To the airport I've done it from as low as 225 Baht (in the middle of the AM). As such I never go 'off meter. Anything more than 400 Baht is absurd. But a meter close to it (or 401) probably isn't a scam. Just a long trafficy ride.