I'm as suprised as you, and I've been around for a while. It seems to be a simple supply and demand problem, but I'm a little suprised at the demand for such old and potentially unreliable hardware. AmigaForever or UAE would seem to be a more logical choice for those with retro fever.
If you look around a bit with some patients, there are better deals to be found. I saw a 4000T with monitor and extras go for about $650 last week. AmigaOnes have never been very cheap though. The best US price I recall when I was looking to buy was $850 and it's just gone up from there in the past several years.
I'm afraid there won't be another mobo maker as long as Amiga Inc refuses to license any more OS4 capable systems. If demand remains while the prices on old equipment rises, I couldn't be suprised if a mobo showed up that wasn't licensed, or even OS4 compatible. (At least before hacking)
Plaz