-From my understanding, with the exception the DA and perhaps the ZDZ, most if not all the other engines are simply conversions of engines made for chain saws or the like.
Again, it's a numbers game. (as well as clever marketing) It's just that those "few mods" can really jack up the price from a tooling standpoint.

Also electronic ignition, something that would be a hassle in a weed eater, eliminates the requirement for a magneto which saves weight and makes the engine easier to start with out the benefit of a recoil starter.

Just some thoughts.