Im surprised a lot of people don't do it with empty older servers and virtualisation, I am sure a lot of people are tempted.
Honest answer, not worth it. Older servers might be 32 bits, which are way slower than 64 bit systems for things like scrypt. And the supporting chips on server class systems burn a lot more power for what they do than cheap-o trashtops.
I remember running Seti@home in the 1990's on a 16 processor NCR4300 with Ppro chips. It was one of the fastest systems on the network, but it was eclipsed in a few years by a pair of P4's.
C
