£450 sounds a fair price for that, as it will be difficult to source a better door I would have thought. Similar cars with rust issues and no MOT would not fetch that, so it looks like Y10 prices are on the up. Though not as up as the owner thought apparently!
Difficult but not impossible if expensive
!http://www.ebay.it/sch/i.html?_from=R40&_sacat=0&_nkw=lancia+y10+porte&rt=nc&_dmd=2
At least two pair available.
Brian
8227