Reminds me about the popular Bose snark: better sound through advertising.
MM had the good luck to have a full line of clone transformers available when the DIY thing was really starting to pick up. They advertise that full selection well, compared to Heyboer, who just don't advertise that hard to end users. They seem to be doing well enough on their pricing model or they would have lowered it, and their pricing helps their premium image, even though most of us here know full well that as-good or better clone transformers are often out there for much, much less.
Yeah, I could hate on them some, but you got to give it to them for doing smart business.
If you do want to use them you should never pay the one piece price on their website. Any dealer that sells Mercurys can get any unit they sell and is allowed to offer it for sale for 20% off the 1 piece price; However, this doesn't mean you can't offer the dealer less than that. Still pricey that way, but better.
FWIW, when I finally get around to my Express, I am going to use Heyboers.
TBH I'll probably use it myself to build another Express clone, but this time using a proper case and following the build guide posted here. I'd also love to fab up a wooden case in the style of one of Ken's amps. But if the price is right for that piece of iron..
Also to add on to what Bear said earlier, I believe the BOSE name stands for "Better Offer Something Else" :lol