How much does a 100 watt handwired amp made in the UK or USA cost these days? I mean, even ones you don't like?
Apparently, they cost $4,000?
The same exact circuit with mostly the same parts and, arguably, even better transformers (Marstran, Merren, Heyboer) can be had for about $1,500 if you know where to look.
The R&D for this mod has already been done. It's on the SLO Clone Forum, Amp Garage, Ceriatone's layouts, etc. And all of the parts Marshall is using are fairly pedestrian: Alpha pots, Belton tube sockets, Cliff jacks, Vishay/ERO MKT1813 signal caps, generic carbon film resistors, and whatever electrolytics they're using (ARS, Ruby, F&T, TAD, JJ, etc.). None of that stuff adds up to $4,000. It's the labor and the branding. And labor can be had much cheaper. So all you're paying for is the Marshall logo and the Green Day association.
It's nice to have it all in an "official" package, though. I get that. It's probably perfect for that die-hard Green Day fan. But under the hood, it's nothing all that special.