I obtained a quote on a Prinns or more to the point I didn't due to valve seat problem. This was in 2006 and extracted the text here
Latest information from Prins NL, our preferred supplier, states that the 20-valve 2500 Turbo (2003 onwards) as used in the XC70, suffers from valve seat recession on LPG. This means unfortunately without expensive cylinder head modifications (hardened valve seats) you will have a very expensive bill looming after conversion. Should you wish me to provide a detailed quotation for the head mods, I will do so, but to give you a rough idea, expect to pay circa £1500.00 + VAT on top of the conversion which would be £1900.00 + VAT.
The info was provided by Alternative Fuel Systems Ltd and although obtained a quote from another dealer advising "wasn't aware" of a problem this did not feel me with confidence to splash out thousands of pounds on a conversion only for it to screw the engine up.