Well, someone must be giving it a go, as two ex UAS Doggies are parked at North Weald waiting to be fiddled with. As well as the rather high sounding figures bandied about for the mod, the various bits and pieces that break and fall off cost a lot because of the military thang, but, as A and C suggests, it may be possible to bring the costs down a bit. You might have a word with Roger Hayes of Skysport who is very knowlegeable on the the subject of Bulldogs and their little ways. He may have a clue as to the prospect of obtaining realistic prices.
Here's an ironic contrast: apparently the US may be an active market for the Dogs, as the FAA is said to be quite relaxed about tired-out hoonds blatting about in the friendly skies. Meanwhile the USAF plans to scrap its large fleet of barely used Slingsby T 67s because they are worried about getting sued if they sell them to people who then crash them (you may recall that the fleet was grounded after some accidents, one of which led a typically stupid US jury to accept that it was Slingsby's fault that the Lycoming stopped in a spin). So, it's alright to let lose aircraft that have been pounded and thrashed by generations of UAS studes, but not OK to sell perfectly good and almost new aircraft, even to us non-litigious Brits.
Returning to topic, if the costs do come down then it may well be worth getting one as they are such good fun to fly. Such a pity that you can't get another seat or two in the back.