RetroGaming

I've casually perused some Gameboy mods over the years, but never actually done any

And I have no idea what these shops are offering

But to give you an idea of what I've seen out there in general

There's of course shells and buttons, and replacement parts to replace worn/damaged components (and I would imagine that in some cases those new components might be better than the original)

There's options for better screens, and backlit screens on models that didn't originally come with them

USB charging (and charging in general for models that originally used disposable batteries, and higher capacity batteries for those that already had rechargeable)

Video output to hook it up to a TV or whatever

Amplifier circuits and such to improve audio quality

And I'm sure there's others, but that's what I remember seeing off the top of my head.

50000 yen is around $315 right now. I feel like I've seen some places selling pre-modded Gameboys at roughly about that price point, so depending on what they're offering that's potentially a pretty solid deal.

But a lot of mods seem like they're pretty much just a matter of opening the case up and popping the new components into place, and the parts aren't terribly expensive, so you could probably shave some money off by DIYing it at home if you're more interested in having the Gameboy than the experience of going to a workshop to do it. Some mods do require a bit of soldering, it didn't look like it was particularly complicated soldering, but I know that a lot of people just aren't interested in attempting that themselves, don't know what your personal comfort level with that is.