cheapest board i've seen with sata6gbps, usb3, hdmi, optical audio is around £75. Sandy bridge boards with these features start at £60.
Only a few manufacturers have their boards on sale or pre-order at the moment though, hopefully others with those features will be cheaper as its a little pricy.
The lowest priced AMD Llano desktop will be around £45-55 which is good value, about the same as pentium skt1155 which has integrated gpu, would be nice to see speed comparisons of those at the end of the month.