Andrew,
If you hunt for them on eBay for instance and select the option for "More Refinements" then "Show only" and tick the boxes for 'Completed Listing' and "Sold Listing", you can see the last 3 months or so listings – 'completed' meaning it didn't sell through eBay and 'sold' where the selling price is shown.
I have used this method in the past several times for a mutually agreeable value for items.
Currently I see one at 770GBP, one at 720GBP and one at 285GBP as well as sets of castings and books.
Simon
edit: I see the 285GBP machine is only part complete, presumably explains the price
Edited By Simon0362 on 23/10/2014 13:51:10