Thank you for all replies!
I would like to rewrite my idea because english is not my native language and so I suppose I was not very clear.
My idea is (numbers are invented):
a) server for my purpose = 50 $ bill for power;
b) server for my purpose with a gpu/cpu/asic/whatelse added for mining 200$ bill for power.
But mining gives me money so if I get more than 200-50=150$ from mining I am better to do B instead of A.
So I do not ask you for a precise calculus; I ask only if you think it is possible to cover A costs using B configuration.
Thanks again,
Mario
That will depend on how much you pay per kilowatt hour (kWh) or per megajoule.
It will also depend on what kind of GPU you will use in the server.
If you pay more than 0.0082 BTC (convert to your local currency) per kilowatt hour, or more than 0.0023 BTC (convert to your local currency) per megajoule, then you will probably find that you will pay more for electricity than you will earn mining Bitcoin no matter what GPU you use. The earnings won't compensate more than the additional electric cost.
If you pay less than the numbers I quoted above, then you might be able to offset the costs a bit, but it will depend on how much less you pay, and what GPU you install.