I know that some other "classic" band t-shirts from the same era, like James' "Come Home" or the Inspiral Carpets' "Cool as F**k", appear on e-bay occasionally, and tend to go for around 10 UKP - and if they're still bagged, it can be a lot more. This might not necessarily reflect the price of an original KLF t-shirt, though. John Not Scots, just Scott-ish.