Does DME actually dyno tune the map to your vehicle? If so, then ok, more money could be justified. If it's one of their OTS maps, then you aren't paying to play, it's just wasted beer money.
The reality is even with BM3, or MHD, a "custom" tune isn't fully car specific. You would need to put it on a dyno so they could run the engine through the entire 2D map to dial it in. The "custom" tunes are most often focused at WOT mapping. That is a lot of hours on the dyno, which costs money.
Sorry, never heard of them until you posted.