How much would it cost to run my desktop cpu to run all night for a month?

Its brand new, core 2 duo with 4 giga ram. I started downloading lots and lots of things from the net. If anyone could tell me how much would it cost to run my cpu only, cpu only, in terms of some other electrical product (like the tv or something) or just in terms of money, or just in terms of expressions like negligeble or ghastly, for a month, all night, then i would be able to decide what is better, to download a thing in 3 days, or just buy it in the shop for some bucks. Just give an approx value.