I would guess they are in the £150-250 bracket, depending on condition, model, and the market. One of those guns that you may find cheap, or you may find an enthusiast seller who asks more.
I think the rarest is the .177" Mk2 carbine. The best is the .22" Mk2 carbine.
Deduct £20-30 if it doesn't have the iron sights.
It does not have the scope mount issues from earlier BSAs. But it has a weird 14mm rail that requires a special BSA mount. I have one of those on mine, but it was a bit pricey and fiddly to set up. Best bet is get one with irons and a period mount.