These guys get it.
If these are yours to do with as you please, including selling them, then your best bet is selling all of them either bulk or 1 by 1 for like $400 or $500 each, since you said they're decent spec.
If you sold 100 pc's at an average of $500 per computer, use this $50,000 to buy Bitcoin, you'd accumulate 220 BTC.
That's more than any mining or service you could ever provide.
Sell it all and invest the money in Bitcoin.
Selling those computers for bitcoin may be the best option. 
If these are yours to do with as you please, including selling them, then your best bet is selling all of them either bulk or 1 by 1 for like $400 or $500 each, since you said they're decent spec.
If you sold 100 pc's at an average of $500 per computer, use this $50,000 to buy Bitcoin, you'd accumulate 220 BTC.
That's more than any mining or service you could ever provide.