i do...

basically the loom that comes is often customisied to fit a particular car so you neec a new one of them to start with. then they don't like second hand tanks as you aren't sure of any potential damage, then all the pipe work must be new so in reality the only bits you can re-use are injectors and the brain and even then injectors with 100K miles on them would be over half worn and thus harder to tune up so the brain is all thats left, in which case for a couple of hundred quid they don't see the point.

most of the cost is the actual fitting in hours and days, if you are fuly competant then you may be able to negiotiate a discount if you fit all the tank/pipework to their standard, fit and locate the vapourisor, electronics etc so all they have to o is wire it all up and test it.