The Gradius board uses Konami's "Bubble System" (
https://en.wikipedia.org/wiki/Bubble_System)
Unfortunately it's pretty volatile and therefore there aren't too many boards still working, so it's pretty rare!
Whilst there were some Gradius boards that use ROMs, they were very limited (possibly sent out to replace broken bubble system boards under warranty), so they're pretty rare!
Not sure which one this is, but that goes some way to explaining the cost.
Nemesis was released on a normal ROM board and is therefore more reliable, so less pricey.
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