I hope the speed limit reader doesn't mistake highway numbers for speed limits. In some areas they are quite similar. More significantly, if the reader misses a sign, it will report the previously observed limit. In many areas this could be a very serious problem (small towns often have speed limit signs obscured by trees so the car will contnue to report an unrealistic 65 mph highway speed limit). Also, many roads continue for a considerable distance without a speed limit sign so the limit will be unknown. All in all, I'm predicting that the GPS database will prove more reliable.
Good plan to reduce the cost. This will help to increase adoption. I like the addition of speed limits using traffic sign recognition. My GPS has the speed limit, but it depends on the database instead of actual, so it could be out of date.