I'm actually doing this... what kind of stuff are you actually looking at?
Those taxi-buses print advertising with photos? or just plain vinyl letterings (eg, Comfort taxis? If it's uncomfortable, maybe we can take this into PM.
Cost depends on a few factors:
1) Design artwork (if you have a vector copy, it'll be much easier. If not, have to vectorize manually)
2) Layers (1/2/3 colour overlays?)
3) Complexity of artwork
4) Installation services (if any)
5) Type of vinyl material used (3M vinyl is definitely more ex. compared to the rest)