I chose the 'pay-per-click' option and what you have to do is place a max bid that you are willing to pay per click. In reality you may be paying less than that bid, depending on competing bids for ad space, but if I remember correctly I put the bid around $.50 per click, and the ad ran TONS of times. You can always adjust your bid later. Also, you need to link a credit card to the account so Facebook can bill you for the ads. You can set a dollar limit per day--I think I set the Symphony's at $20--and most often you will not be anywhere near that, as you only get charged when people actually click on the ad, as opposed to paying just to have it show up a certain number of times.
I would say that the ads had a modest effect on the number of Facebook fans the Symphony gained. If you really want people to join, make sure you put the "find us on Facebook" logo on any print ads you may do, too.