Desktop computers use around 200 watts per hour while in use so if you run a computer for 24 hours at 200 watts per hour the desktop computer would consume 4800 watts per 24 hours.
If your electricity cost is 10 cents per hour it would cost you around 50 cents to run your desktop computer for 24 hours.
Most times though a computer will go into sleep mode where things such as the fan and hard disk will shut off so your computer would consume less power if those aren't running.
Still if everything were running on your computer it would cost you only $15.00 per month to keep your desktop computer running 24/7.
A laptop computer will use less power and would use only around 12 kwh of electricity over 1 hour or 83 hours of use for 10 cents so you could run your laptop computer for 651 hours for just $1.00 which is very cheap.