I have needed to build several computers just lately which have been connected to the TVs in each room I have also upgraded the comps in the computer room, 9 or 10 in total so cost has been a big issue. I have managed to pick motherboards, graphic cards and AMD+ CPUs up from Ebay ans HD and memory up from Aria. I have used various speed CPUs depending on what they were being used for but most used AMD 5200, 2G memory, 2 x 1TB HD (RAID), various motherboards and all but 1 cost me less than £150 to build (inc case). One cost me damn near that much just for the case, a Thermaltake Bach. They are certainly faster than the 32bit IWill MPX2 motherboards I have just retired and the MPX2s were faster than any single core computer I have used.
. . yes; interesting that it is only recently that fairly ordinary retail dual-cores have improved on the overall performance of olde-worlde dual-Athlon boards from half-a-dozen years ago.
My sister - a professional photographer - is still using a dual-Athlon mobo, with 2GB of registered DDR & a modern~ish 250GH HBB - & the thing is still pretty brisk, if noisy by present-day standards.
£150 a PC is pretty good going . . . . . if using Windows this presumably means that the cost of the OS equals the cost of the whole of the rest of the PC . . . .