I think it's a neat job.
Room for improvement on the power side though.
As for price, it's stiff, but he's probably put a lot of tlc into it.
For parts:
CDROM: Can be found cheap
IDE HACK: Cheap to expensive, I suspect cheap here
Power: From £50 to £100
Screen: From cheap to expensive
Speakers: Cheap
Amiga1200: £50 to 100
Blizzard: About £100
Indivision: About £100
Os39: About £20?
Disk: Cheap
So lets start by saying there is parts for £320 to £420
Then there is work, metal parts, plastic parts and the tlc...
If I built it, I'd be proud too, and asking a lot too.Sure his premium is high. But we do not have to bid.