Should i buy Burns monster

It's anyone's guess really. The Burns Monster doesn't do anything except walk around your town like the Mayan God and the Krusty Doll, but there's no guarantee that your treats will have any value if you don't spend them.

If I was in your situation, I might hold off buying the monster just to see if they offer another item at the end that may have better value. Hopefully they'll give you a warning that the items are leaving the store, and at that point, if nothing else has been added, go ahead and get the monster.