CAT C details are actually recorded on the V5 now, (please no-one say they aren't because I had one not long ago) so I think that'll always mean they command a lower price.

I've bought several CAT D and C cars in the past, they need bits of work but most of them are better in the end than a lot of so called 'good' cars that are for sale, because a lot of the ones that have been put up for sale are to try and get rid because they have problems, insurance writeoffs were probably usually cars people had no intention of selling til they were written off.