Collectable items are only worth what the buyer is willing to pay, regardless of what the item actually is.
Fred Fish did play a HUGE part in establishing the critically important freeware and shareware aspect of the Amiga long before the Internet was available to mere mortals, long before even BBS's were en vogue, and even before 300 baud modems were within the reach of the average user.
Fred Fish was the source of Amiga software for thousands of Amiga users when software wasn't easy to come by. As such, I do see a little bit of collectability there, but to me (personally) about $100 is pretty much the limit for what would become a closet-filler.
Several people who might have a collectable Amiga fall into a much more desirable category... Carl Sassenrath, Dale Luck, Mike Sinz, Dave Haynie, Jay Miner, and the other original developers come to mind. These are machines owned by innovators that bear serial numbers in the tens, not thousands.
Then again, I have very little available closet space.