Glyfair
Explorer
The issue with this is that plastic miniatures have a very high upfront cost. The costs are kept relatively low on plastic miniatures because of the number printed. Given the low print run of the set, as you concede, you would not be spreading that cost very far. A "cool unique" monster could easily cost the $75 you want for the set.But, if you're going to make a cool, unique monster that's only ever been used in that adventure...then make a cool, unique mini to go with it in the boxed set. Cool, unique minis are things that DM's love...something that could get them to buy a $75+ box of minis...especially if the adventure is good.