Why does this cost more than the carts for, say, GBA?
Look inside one of those GBA cards sometime. You won't find much. They're mass produced in huge volume, with custom ASICs, and benefit from an inherently simple design. The 64drive is far more complex than these, both because of parts costs and efforts to provide a polished product. Trust me, I made it
The price is well justified.
I can play all these games on my computer
That's nice.
I wouldn't pay more than $40 for this.
You probably wouldn't buy it anyway.