Okay, your question is unfortunately vague. what it CAN sell for ... is what ever you can get and there are an ENORMOUS number of variables to that.
Certain things can increase the value, such as having multiple email lists with leads/prospects in one or several from various sources for different purposes, and then various paying customer email lists. Having something like that can increase the value A LOT.
Also, what about your current competitors? Could you create something of a bidding war with them? (if so, more for you)
Do you "need to sell fast"? If so, much less for you.
Consider getting a business broker and see what they can do ... but off the top of my head, if you need to sell fast, look for maybe one years worth of earnings. If you can hold out for a good deal, then you could climb that up to 3 to 5 years worth of earnings. Just remember that you have to pay your business broker out of that (about 20%)