I wouldn't focus too much on likes on FB any longer. In the past FB pushed promoting likes as a way to get "free" reach. Now that they have butchered the "free" reach you really need to pay to play.

From what I've seen under $1 per like is good, but you can get as low as 20 to 30 cents in a really well targeted campaign.

If you're going to advertise on Facebook, make sure it's for the purpose of getting something out of it more than a LIKE. The problem with advertising for LIKES is that you will need to spend additional funds advertising to the people who clicked LIKE on your page because Facebook has drastically reduced the number of fans who will ever see your posts on your page, so the best thing to do is concentrate on building your email list by advertising on facebook for the purpose of driving your traffic to a landing page or for sales, driving that traffic to your website. You will likely capture a bunch of page LIKES from the people who are being sent to your landing page or your website and those LIKES will be free, so in a way you're getting more bang for the buck when you advertise this way. Just remember, Facebook is a lead generation tool and nothing more so you have to make sure you're using it that way.