Hard Off and Book Off are great if you want good prices. They are thrift stores, and tend to price with that in mind. Of course rarity does apply but I’ve never spent more than 3000y on a single item at my local location. Pricing does differ from place to place, and the rural ones tend to be cheaper.
Also from my memory, the Akiba Super Potato are way over priced, but the one in Ikebukuro tends to have better deals.
Does he have expensive night vision goggles? For night jiggling?