Please define best

Is this supposed to be most productive ?

Am I supposed to consider electrical use ??

5 Odroid xu4's @$79 each running Android. 8 cores each means 40 total cores. Case not needed unless you like pretty things. 16GB micro SD class 10 cards to run them are cheap. Electrical would be next to nothing too

If you're talking PCs, you can get used blade servers on ebay running dual hex core xeons for sub $500 price.

I would buy 13 RCA 7" quad core tablets at wally world for $38 each, $494 total.

13 X 4 = 52 cores @ 1.2 ghz

The chargers max at 5vdc @ 1 amp ( they never use any where near this much ) for 5 watts apiece or 65 watts per hour.

52/65 watts = .8 watts per core maximum

65 watts X 24 hours X 30 days per month = 46.8 Kwh

46.8 Kwh X $.20 per Kwh = $9.36 a month maximum possible electric cost.

Once originally charged, they only need a small trickle to maintain the charge so the usage and electrical costs are much lower than above.

With just this in mind and presuming that secondhand equipment is allowed I find that the argument is going to be between more older slower cores or fewer newer cores that tend to run faster.

To put things in perspective and miss your $500 target for a moment..... Back awhile I looked at my overclocked 2600K and what it was achieving. At a certain overclock I could achieve around 9000 boinc points per day on one project but doing so was using 240w from the socket. The chance arose for me to get a dual socket xeon e5 setup that ran at half the speed and with 4 times as many threads and as you might expect it achieved twice as many points whilst still using 240w on the same project.

If you can stop laughing for a moment when I say that points are related to the amount of work done then it seems to me you should just look for which machines produce the most points and work your way down the list until you find one that you can build/buy for $500

OR

In view of your comment "electrical use doesn't matter" maybe consider even older cheaper tech but more of it. You can probably buy 2 or 3 xeon x5550/x5570 dualies for the money (More if you use blades) which will do more work but each one will likely use 250w plus

copy these results into your own spreadsheet and sort on ppd to get you started:

https://docs.google.com/spreadsheets/d/1ll78D...9Xazh6Bvo2xlII/edit#gid=0

As a general rule work done is proportional to watts used. Just remember that each generation of xeon cpus is more efficient the newer it is and that is probably why companies upgrade their servers.

Priorities seem right to me but although CEP states 1GB per thread is required you can read that as including the OS for the most part (8t plus)

When building crunchers it is preferred to use Linux anyway (free and uses less resources) so should be good with 4t and cep. Again just for crunching it is not difficult to set up Linux