I don't know if anyone has better prices, but Eyetech has (had?) AmigaOne-XE motherboards starting at UK£528.75 incl. VAT. That board has a G3 750FX @ 800 MHz:
http://www.eyetech.co.uk/search.php?SearchStr=&SearchCat=AMA1
They also sell prebuilt AmigaOne in a few configurations, starting at UK£900 for G3 @ 800 MHz, 512 MB RAM, 40 GB HD, 52x CD, Vibra 128, Radeon 7000VE 32MB, Ethernet, USB, ATX tower, Debian Linux PPC and OS4 enabled BIOS.
Apparently, AmigaOne now are manufacturing MiniITX sized µ-A1-C with similar specs to above, except the sound is CMI 8738 and only one PCI slot (dunno if they had more before). The card is from a company called Teron who seem to have been liquidated and Eyetech is said to relabel and sell the remaining cards as AmigaOne. I can't find any information about this on their own website though. Once these cards were sold out, no new AmigaOne cards seem to be available.
A Swedish dealer who currently does not sell AmigaOne, since the motherboards are unavailable:
http://www.ggsdata.se/Amigaone/eninfo.html
An early article on AmigaOne:
http://elwoodb.free.fr/articles/AmigaOne/
A more recent review:
http://arstechnica.com/reviews/os/amiga.ars
To add further confusion, there is another project called Amy, which is a competitor to AmigaOne by another company, and AmigaOS 4 will be ported to run on both platforms. Amy'05 is supposed to be more available than AmigaOne. There is also another PPC based platform called Pegasos, which I'm not sure how Amiga compatible it is or if it is commonly available.
Conclusion, it appears to be a mess and you might have to look around to find a motherboard at all, not putting emphasis on how much it costs.