Good point, but Cableshaft had the right idea of it. I mean, at no point in the plumber scenario does the plumber have to actually part ways with his weed: "Sorry officers, I guess the guy must have moved his stash after he realized I must have seen it!" He didn't take custody of the drugs he's reporting, he just took a picture / called it in.

In the dry-cleaner scenario they obviously wouldn't have a great excuse for missing dime bags since they'd have custody of it.

REGARDLESS, I think the point isn't about "setting the right price" or comparing scenarios in which planting evidence is easier / harder, it's that ANY price is going to enable perverse-incentives for bad actors in at least some situations.