On ebay the 88 (not plus) goes, on average, between £200 and £250, and the plus models tend to go between £250 and about £300. Obviously this depends on age and condition and a bit of luck
You can get them cheaper as ebay prices tend to be higher than the for sale pages on this forum and other 2nd hand places.
I`m after an 88+ myself but I`ll have to wait till I get some more funds