About how much does it cost to leave 12 led bulbs 60watt replacement burning for 24 hours?
a "60W replacement" LED is usually around 10W actual. 

10W * 12bulbs * 24 hours = 2880Watt-hours

2880Watt-hours = 2.88 kilowatt-hours

Your electric bill shows the price / kilowatt-hour. For me, with all applicable taxes and stuff, it's about $0.145 / kilowatt hour (I just paid my bill, so I have it right here)... yeah, that's 14.5 CENTS.

So every 24 hours those lights are on would cost me:
2.88 * 0.145 = $0.42

So if you were to use a timer to run them only 12 hours / day, you'd save $0.21 / day. If the timer costs $20, it would pay for itself after 100 days.

Cheers,
CList