W&W Dungbomb Dept
The Dungbomb is also a well-known prank item, and the muggles call them stinkbombs, but they are taken to new levels by Fred and George Weasley. In fact, it is the main cover used by the twins while filching the Marauder's Map from Filch's chambers.
Further, the students rebel against the dictatorship established by the Ministry under Umbridge, by simply throwing a large number of these items whenever they could. The original cost a Sickle and 21 Knuts, so it can be assumed that the price hasn't changed much at Weasleys' Wizard Wheezes, either.
So get proficient with your bubble-head charm, because the newest employee in the Dungbomb Department will probably be testing all new concoctions on you.