I think the contents of the wallet are too much like junk without the money. Some business cards, a grocery list and a key in a transparent wallet. To me that's almost as junk as finding some fliers and I might think somebody has just thrown it away as litter and $13 is like lunch money. So it has to has $100 so I think "they might want this".

If it was a regular wallet full of useful cards, perhaps some sentimental things like photos etc. then I'd want to get it back to them regardless of whether it contained some cash. I think this experiment might work better with a backpack or a mobile phone.