I don't think it is because the people are rich, it's because they are now at the age when they have both time and money to invest in their hobby.
I've seen this happen in the past with other things. In the early 1980's I was in a car club devoted to 55-56-57 Chevrolet cars. I was very young at that time, but everyone else was in their early to mid 30's. When those cars were new, they were kids too young to afford them. Now that they were older and their kids were almost grown, they had the time and the money to invest in the cars.
It also happened a while back with the full-sized arcade video games. The kids that grew up in the arcades, including me, were now old enough to buy and restore them. The prices went way up on them for a while and have now started coming down for all but a few of the most popular machines.
Now, think of someone who wanted an Amiga but couldn't afford it or had one back then but couldn't afford the nice upgrades for it. They are now old enough to have completed school, have a good job and their kids are old enough to take care of themselves for the most part. These people now have the disposable income and time to put into building an uber-Amiga and are not afraid to spend the cash to do it.
This makes it hard for everyone else that is not in that group. Eventually the prices will adjust themselves to more affordable levels (like the arcade machines) or the hobby will stagnate (as has happened with the old cars).
Only time will tell which scenario happens.
-JM