There are lots of rules of thumb, but it depends on where you are, what type of system you want etc. Is it a large system or a small system, a one off room or several rooms...is it a chilled water system, a DX system etc
For one off rooms, a simple 'split' AC unit would suffice at around £3500.00 inc wiring
for a larger system, say a chilled water system with ducted fan coil units, etc, it may rise to £4500.00 per unit or more.
Also could you use rapid fixing methods such as 'Mapress' or plastic pipe such as 'Aquatherm', or are you restricted to steel pipe and screwed or welded fittings.
How long is a piece of string???
Friar Tuck of Sherwood