An intensive study probably needs to consider many factors but if you are looking at straight cost of mobo/cpu and calling the rest (power/case/floppy/video) some $ constant
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then it gets a little easier.
I've been keeping this very topic under review as my home cluster grows directly in proportion to the bang/buck model for new hardware.
It is possible to buy discounted LX motherboards and run Celerons in them. This generally gives the "cheapest" option and probably a fair output of say 1.6Mkeys for a Celly bubbling along on Klinux at 566. If the $ is the absolute limit then see Russ's crackrack instructions.
I opted to buy video on board Asus P2BZVM mobos combo'ed with Cel 566 for my original Klinux nodes. It was too easy to mount them in cheap cases and use old floppies and 32MB RAM sticks to get them going. Output 1.6Mkeys. I haven't been clever enough to o'clock them yet (Jump in with hints and instructions anybody)
For about $5.00US a unit more though, I have been able to get ASUS A7Pro and Duron 600 combos.
These are seriously easy to O'clock to 800. They do need a fan (golden orb) and a video card (2nd hand 1MB) to run though and this extra cost has to be put into the equation.
Output - a whopping 2.9Mkeys!

If your looking at bang/buck then this is my tip.
I intend to sell the Celeron nodes and replace them with Durons. Just doing this will give me an extra 3.9Mkeys and at worst would be cost neutral.