I know this is old but I haven't been around for a while.
Personally, I don't think Donkey Kong is rare. In fact I have a sealed one and I don't even own a 7800

The thing is, with games like Donkey Kong, people have a certain "connection" with it, be it fond childhood memories or just general pure nostalgia. As with a certain Mario, Donkey Kong has become an icon, and the game has defined many a gamer's roots. See, while a 7800 Donkey Kong might not be readily available in the UK, Super Mario Bros. on the NES runs up ridiculous prices here in Denmark. Most auctions end at 100DKK+ (That's £12+!), and that's just a loose cart.
I know the NES was hugely popular here, as was the Sega Master System. The 7800 was pretty much nowhere to be found, which is probably why NES games are outrageously overpriced. I guess the 7800 had a broader audience in the UK, thus creating a larger player base and thereby increasing the number of people attached to the Donkey Kong game?
This is all pure speculation, mind, so I have no idea if this is the case at all. It just kinda makes sense - well, to me at least.