As you said, it is supply and demand, there is no need to invoke hoarders (although from what I read on the EAB ere are quite a few cases of hoarding here
but it is anecdotal evidence and hence irrelevant).
Most of existing Amiga are in inaccessible places: attics, storage rooms and landfills.
The few which are on the market are not enough to satisfy the existing demand of people who realize they should never have sold their old one or just want to discover the machine.
n Amiga available, n*m people who want to buy (m>1)
=> the price goes up
But if you want cheap Amiga hardware there is a solution: go knock at all the doors of your neighborhood and ask if they have old computers they want to get rid of for a small price. If you do that for a few weeks I guarantee you that you will find a lot of treasures.