Everything is worth doing if you do it right

You are referring to the promoted post ads, it's a one time cost, no additional increase in budget is a strange way of saying that but that's basically what it is.

As for effectiveness, they're as effective as a giveaway, yes you get traffic but its not so targeted.

The effectiveness of these posts is they target friends of people on your fan base or friends of friends etc and display that, so when someone see's it, they already see a friends liked that page or post and are more likely to like it as well.

This is gold for internet marketers because we are consistently networking and friending other marketers which means you get more exposure to the same types of people who are also marketing online.