To offer my own personal experience, I bought mine less than a year ago for £8k knowing it needed a lot of mechanical TLC (but otherwise in pretty good nick), £5k later I still have belts and clutch left to do.
Having gone through that, £15k seems a fair target for something that's had...
Looking for the elusive 6000073748 for #399 and, whilst I am also scouting some of the usual sources, thought to post here in case anyone finds themselves having one too many in their war chest :)