I've browsed the net to search for Facebook fan page of businesses and found lots of fan page that offer advertising your business facebook fan page through their "wall/Ladder Time". Are they really helpful? Take note that they only accept "Facebook Fan Pages" and not your homepage URL.
You would need to see how their target market fits with your target market and then the cost per page member. If you are selling a $100 item and they are charging $1 then you need to work out how many of their followers are likely to buy and therefore what your cost of selling each $100 item really is.
It still a new thing to me, so I'm still in the process of studying how this kind of advertising works.
Its no different to any other media Lets say you wanted to advertise in a magazine. It would have, say, 100k readers. Of those readers you would expect to get 1k sales. You then look at your gross profit to see if you can afford to advertise in the magazine. Basic business calculations really. Don't let the fact that its facebook make you think its cleverer than it is.