I remember a SAR article that purely fawned over the HK 21e. I was convinced until I looked up how much one was. I would wonder how many HK sear owners even own an HK 21 host. I have no real idea but I'd bet less than 25%, so you are getting to a pretty small pool. You could learn a lot about how much people will pay by looking at what the market will bear for HK hosts and M16 uppers. I figure there weren't many 21 hosts being made before the HK sear reached a certain price point. that MM 21e is what, 15k? If someone spends 40k on the sear they might go for it, but that's 37.5% of the transferable's value.
What's worst is that MM21e gun has a good chance of not even working. Look how many have been listed on hkpro with the following description: Under 1k rounds fired with more semi than full and just back from Mike Otte for service. I know a few sear owners with guns built on fmp or german parts kits. I have a 21k myself. The non e guns are cheaper(around the price of a MM e gun) but yea I have multiple mgs that were cheaper than my 21k.