On average, the weekly cost of a signature campaign is at least 1 BTC, so for a month it is about 4 BTC. Logically, why spending such a sum of money or more for something useless and has no input for your project ?. Simply, because it presents an effective method of advertising.
Where are you getting this figure from? I doubt there's any campaigns that pay out anywhere near that other than maybe Chipmixer (though they're certainly the anomaly). Campaign payouts are all different but for the vast majority of alt coin campaigns they pay you in the token they've created out of thin air for free so it doesn't actually end up costing them anything. The vast majority of them also have campaign managers who do little to nothing if they even exist at all so there's no expenditure there either.