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.
Those days have gone when campaign owners had to pay 1 btc and more. If you calculate btc in usd, 1btc was pretty normal that time and can say the same about 0.2 btc, price rised a lot.
As hilarious mentioned, there is really only one anomaly in payments here (anomaly in a good way). Seems they continue to follow bitmixer's signature campaign rules with old payrates and almost the same structure and aim.
Logically no one spends huge amount of money for months if he/she doesn't gets benefit from it. Just look to primedice for example, they were first with massive sig campaign if I amn't wrong and now you see how popular they are.