Actually, it depends on the payout of what you are promoting. It would be different if it's an email or zipsubmitt, or if it's a clickbank product that pays out $30 or $40.
I do a fair bit with Facebook, and it's usually pretty inexpensive.
When you are using Adcenter, or Adwords you are plugging in hundreds of words that people search on, and seeing which ones of these key words get you sales. So you have 100 words at .05 per click. You need each of those keywords to get a decent amount of clicks, then see if those clicks convert before you start to eliminate them. That can take a lot of money.
Facebook, you can set up one ad, then click the similar ad button and tweak something like, and interest, male/femal, age range etc. Then create another and another, sometime changing the pictures as well.
Then you have 10 ads trying to convert the same product. You start off with CPC and which ever ad starts to give you the cheapest clicks, is the one that you stick with, but each click will count towards your overall campaigns EPC. Meaning, if you get 1 sale, per 100 clicks in facebook it doesn't matter if it's over 10 ads or 5 ads.
Typically I find one ad, with one picture that gets a nice CPC. After 100 - 200 clicks if I don't sell anything, then I change the landing page, the wording in the ad, or throw the campaign in the crapper. If it's an email or a zip submit, I will change up every 50 clicks. Once your ad is getting a decent CTR then you can change to CPM.
Now you are making money....I hope.
Hopefully that all makes sense.
-Capp