We went thru the PRNAV process 6 years ago, the approval in itself was just a matter of showing our CAA the appropriate documents and wasnt very expensive, less than 200 Euros if I remember it correctly. We did this after we had made a major upgrade to the panel (100kEuro), since it was a major mod we already had all the documents they were asking for. And since EASA requires a typerating and HPA for the PA46 we as pilots were already approved since the HPA included it... So all in all I believe the approval was a nonevent, uppgrading the aircraft however was a whole different story with avionic shops trying to dig really deep into your pockets by not telling you the small things like the fact that you can get a PRNAV approval flying manual so you really dont need that approved rollsteering computer for the AP...
By the way this was in Sweden, UK CAA I have no experience with.