In a nutshell: ALOT

{ if you want to skip the diatribe, go to the last sentence}

My reasons for saying this:

1. Since you are having "problems" you most likely have a leak somewhere (I think I read on a past post that you do) If you need a new evaporator, they are not cheap.
2. The new refrigerant runs at higher pressures increasing you chances of A. developing a leak, and B. wearing out your compressor prematurely. (fortunately you have an R4 comp. which is not so outrageaous as other types)
3 Unless you go to a professional with a concience, you could end up with an abortion of a conversion like I did on my deville...three months after "conversion" the compressor gave out, costing me $800. (PS I did not own the car at time of conversion!! [don't want anyone giving me a line of sh## for not going to a pro ])

On the one hand your compressor will be cheaper than mine, but on the other hand other parts will most likely be more...and then you have to see about where your leak is.

In short, go to a reputable A/C place with experience in MB's and have them do a thorough check of the system, (using dye, not the sniffer) and see what they say...then get back to us here, with what they've told you.

-Larry