That’s a double sided sword. It depends on so many other factors other than how much you spend on ads. Content is king. If your page has engaging content people are interested in, then you will get more. If not then don’t expect your retention rate to be very good.I hope this helps. Cheers!

Great question @fernandocortes!  So many people want to know the ROI (especially of social media) and it’s not always clear or easy to measure.  
I think there can be a couple of factors.  You need to know what percentage of your fans buy things from you when you put a sales message up.  Have tracking in place that you can track sales from Facebook vs other platforms.  If you are getting people to sign up for your e-mail list, as well as like you on Facebook then that $1 spent on the fan might be even more valuable.  I track what percentage of my e-mail list buys from me on average.  

When you know those figures then you know how much you can spend on an ad campaign.  The problem with all the return on investment tracking is that it takes time and usually people don’t want to put in that time to figure it all out.  

But if you are asking a straight benchmark question on how much people pay for fans through ads, that figure varies widely by industry and page.  I know people who have pages like “I love my dog” and you can advertise those and get tons of cheap fans because it’s easy to target and easy to click like.  But there are other industries where even with optimized ads you can pay $2/fan.  I recently ran an ad and paid about $0.70/fan.  So you have to experiment with ad optimization and what works in your industry.