One time we were driving down a crowded downtown street and my husband started yelling "get out and pick up that money!" He basically shoved me out of the car to pick up $45 off the street while we were stopped at a stop sign. I was kind of just in shock about the whole thing, so I picked it up and we drove away, but I felt so bad about it! He totally had the finders keepers mentality, whereas I didn't like that we had no context for what happened and the money could have literally fallen out of someone's pocket 10 seconds before I grabbed it.
Since OP has context, I would definitely try to find the owner first. Seems very likely it belongs to someone related to the nearby houses. Though weirdly my gut response was to pick finders keepers in the poll before reading OP's post and remembering my own experience.