I finally did it. I rationalised it by thinking it's not ever going to get any cheaper, and I have the money right now, but I just spent close to £200 on a video game. That's pretty f*cking crazy. I mean technically, it is an asset. If I was suddenly in need of that kind of money, I could just resell it. It's not like I'm buying a wardrobe or something.
I think I got a fairly good deal as well, a loose card sold for £100 that I was willing to drop £70 on, a complete copy went for £310, and my copy has the case and manual but no cardboard sleeve. So I don't know if I paid £100 for a case and manual, or saved £100 on a cardboard sleeve. Either way, that's the final Supergrafx game I needed, and one of the last remaining PC Engine titles I was after. I think there's ~5 more that I had an eye on but obviously they cost a lot less.