maybe there is a good idea behind it?
see the size of the room, power/mounting requirements,
and then quote you.
now if one were to do the installation oneself, maybe a better deal is possible.
It is not difficult to work out how large a room is for anyone with even basic math skills. Considering standard ceiling height is 2.7m, it is easy to then work out the Width and Length of a room and multiplying the two figures to get the square meters (not cube). Most aircon sites assume standard ceiling heights and only work on square meter.
Then you can determine which unit would be sufficient for a particular area.
State whether mounting brackets is included or not, and if not have an option to add mounting brackets.
How difficult is it to state that any aircon above 12000BUT must have a dedicated circuit breaker which must be installed by a qualified electrician to do the coupling?
For this reason I don't agree that there is a good idea behind it.
You are saying that the assumption should be that everyone are idiots and thus should not even have access to pricing on the web?
Doing the above is not rocket science. Really.