@Maxy:
Gag manga typically don't sell the same as straight titles. For a gag comic, Isobe sells fantastically (for points of comparison the other successful go-tos here are Kusuo Saiki and, of course, Kochikame)
EDIT: aw snap, YOKO! Love his work, and his last Jump series was great, despite being a weird mini-series that tried to sell people on the career of comics editor
Comparing with Kochikame isn't really fair either! You're talking of how much Kochikame is selling now that it has more than 150 volumes, but in its peak it was selling way, way more. Basically it has 194 volumes and for years(Since 2007 at least) those volumes sell around 30 to 50k copies each, but Kochikame has an average of 800k copies per volume!
Depending where and how long the decline in volume sales got, Kochikame volumes in the past can have even been able to sell more than 1 Million copies per volume in a year.