does not make any sense as traffic is paid for usely for a 1:1 ration meaning TX and RX should be the same price all around
No that is not the way transit is billed your provider may bill you that way just to keep it simple. If you are being billed by the Mb one of the 2 methods below is the standard.
Transit is billed either at a flat fee for x amount Mbs dedicated port
or you have a burstable port @ 95% of in or out which ever is higher