The SAM board costs about £400 depending on the version and speed. The rest of the £620 is the OS4 license (about £100) and for the rest of the system's hardware: case, drives etc.
Yes, pricey compared to commodity x86 hardware produced in 10,000 batches. However, ask a retailer how much money he makes selling an x86 mobo and then how many he would have to sell to stay in business.
The price you're quoting would pay for not one, but two fully kitted out quadcore desktops. The retailer might have to sell a lot of boxes to make it worthwhile, however, and this is key: He will have customers willing to plunk down that kind of cash for that sort of hardware.
There are maybe 5000 Amiga users left worldwide.
With the greatest of respect to all involved in SAM/OS4, I would have to strongly recommend getting a PC costing half as much and getting a copy of AmigaForever, which will run far more software much more quickly.
--edit--
@cv643d, no, I think you'll find most of us on this board are or at least were Amiga users, not "Amigans". Many of those self identified "Amigans" left this board many years ago in a huff to start in another place, which in turn helped perpetrate fraud. Some of the more insane ones then set up another board and have since been viewed by almost everyone else as a cancer.