Many people would have kept it, I applaud the OP for returning it. Nothing wrong with returning it to the owner. I would surmise that there are more people on the streets without drug money than with drug money. Besides, they toss their DRUGS, not their CASH. Try to remember which is illegal to carry.
20 years ago I found a Seagrams bag by the side of the road. I picked it up and carried back to my apartment as I was near it when finishing my run. It had $700 in cash, a parking ticket for a gold Acura, a few rocks in a small tylenol vial, a small bag of pot, and a list of names and phone numbers.
I called the nearest police station and told them what I found and that I was bringing it in. The detective took me to a room and asked me questions as he went through the contents and then asked me for address, phone number, etc. There was a tape recorder running the whole time.
I didn't care. I hope they busted that prick. I ran and drove on those streets all the time so one less impaired motorist in my neighborhood was fine by me.
I thought about keeping some of the cash. But I didn't. Maybe the detective did. I figure that was his bonus for the week for dealing with life's trash so I don't have to.