Very nice! It is listed in the American Air Mail Catalog latest edition (7th) as Contract Air Mail Route 24 flight number 24S1f. This was the opening of the Chicago to Cincinnati route by the Embry-Riddle Company. The schedule for service of this new route was one trip south and one north daily. This cover has the less common Chicago Air Mail Field cancelation as opposed to the common city pots office cancelation and so this one has a catalog value of $20. The two pilots on this leg out of Chicago were Homer Rader and John Skoning. A complete set for this inauguration of service includes service from Chicago, Indianapolis and Cincinnati - both south and north bound and includes cachet color varieties and airmail field and city post office varieties. The total number would be 8 covers. An interesting aspect about the covers that were flown in the other direction on the legs from Cincinnati north bound is that aircraft ran into a severe snowstorm about an hour north of Indianapolis and was forced to turn back. The pilot was then unable to locate the unlighted airport at Indianapolis and made a forced landing at Friends Woods which is about 10 miles south of Indianapolis. That mail was trucked to the Indianapolis post office. Apparently some of the mail was sent to Chicago by land and some was sent by air the next day. The way to tell for sure if you have an interrupted flight cover is that it will have a Chicago back stamp cancellation and those are worth much more than ones that are not back stamped and that likely went by ground.
Your price paid of $5 is very good, though more for the first flight cover value than the stamp value. As was mentioned, first flight covers and regular air mail covers with this stamp are sold on
ebay regulary for prices in the $5 to $10 range, and sometimes a bit less or a bit more depending on other factors such as the condition of the stamp (is it a bit faded like the one on this cover, does it have a straight edge, is it poorly centered, does it have pulled perforations, is it heavily cancelled, etc.