On a side note: It is kinda ironic that a non-gambling campaign (Chipmixer) is currently offering the best paying signature campaign, by utilising the most lenient model with regards to post requirements. When I say lenient, I do not mean regarding post quality, but rather post frequency. You wouldn't think that a niche service like a coin mixer could "outbid" mainstream services like casinos, when it comes to signature campaigns.
I think you underestimate the volume of mixers: Bitmixer for instance
processed 65,000 BTC per month. I'm not sure how much they charged, but at just 1% you're looking at 650
BTC revenue per month.