What makes it that expensive is the 256 mbits of ram. Slot 1 card don't have that. It is the main reason. The M3 SD pro doesnt have that and is a lot cheaper.
Even the m3 cf ( if you can find it) still cost a pretty penny, probably more than a slot 1. Luckily it's still a great card and works just fine. Add the fact that with an old f2a and wrg dumper it can dump ds games properly.