Had mine done about 3 weeks ago... one of these mobile chappies who at the time seemed quite good... 2p per gram for the gas (750g), £10 for a vacuum/leak check, £15 for a compressor service and £25 for the labour + VAT comes to about £75/£80... He only charged me £60 for cash .... but, although he did a vacuum/leak check he didn't pick up the fact that the condensor unit has a very slow leak hence 10 days later my aircon needed regassing again. I had him back out and again he couldn't find a leak but diagnosed the fault as a low pressure switch (which does indeed cut out the compressor if there is not enough gas in the system)being faulty thus it was cutting out the system even though it was full of gas... he added I'd need an Alfa man to fix it and to becareful because removal of the switch would let out all of the gas and then he went away.
Believing it to be the pressure switch (I think I posted something about this at the time - the compressor would cut in for a few seconds and then stop and it would repeat this every 5/6 mins or so..) I bypassed the switch with a piece of wire and forced the compressor to stay on but it still wouldn't blow cold..... so I took it to a Bosch fuel injection specialists who charged me £25 for a full system health check...and discovered (to my dismay) that the condensor unit leaks...suprisingly enough the most expensive bit in there (£168 + VAT for the bit plus a couple of hours to fit.. + more gas etc.... about £400)...so £90 later and I'm still sweating with the windows down !!!.. if I can give you any advice get the system pressure and leak tested before you pay for any gas etc particularly if it hasn't been charged in a while..
Anywho....after all that I'm still after a condensor unit for a 96 WB'd V6...
PS The charge for diposal of gas must be for emptying older systems that use R12... had a Lancia with R12 based climate/control .... need to find a friendly aircon guy who likes cash deals to get hold of and rid of that stuff these days......melts clouds and stuff they say !!!!
Yes get a full check done had mine done and they found a leaking seal and then damaged the hose connection during dismantalling. So whole cost came to $700 nz dollars. and it is still not as good as my daughters bmw 3 1.8. But reluctant to spend any more money on it.