RFID's are not restricted they have been used to tag and track food stuffs in transit from let's say manufacturer to retail outlet, they are low cost and can be attached to the back of a mobile phone as a a pass if you like.
What maybe is costly is the reader which in this case you would want to subtract a specified number of journeys before the tag is no longer is functional with a particular traveler. Plus the fact that you could pay for the journey before you board a particular kombi.
They are easily replaceable once expired and would not be as costly as producing a plastic card neither would they litter that much.Any way that's just my 2 sense doesn't really count.....