I'ts really hard to say.
I'm only rocking out 4 x 14w CFLs and the electricity was probably something around £1/day when they where on 24/7. As I said though It's hard to tell.
There are lots of ways to find out though. my dad doesent beleive me when I say it's about 60p.
Find out how much your electricity costs kWp/h (1000W /hour) I think mine is 17p. So with 100W of lights for example they would need to be on 10 hours before a full kwp/h is spent.
My figures might be way off, but that is pretty much the jist of all I know about the cost
For sure my PC will use more electricity in 2 hours than the lights will do in 12. 350W PSU. As this is a forum, others might use the same 'measuring stick' of a computer. So if you wanna save alittle on the bill cos your growing, just be energy concious around the house. If you arn't already, and you do a good job, there might not even be a difference in the bill/maybe even less. Don't put things on standby, turn your pc off at the plug aswell as shuting it down, etc etc.
CFLs are electricity shy to put it one way, dont worry about your bill.
Hope this helps