I'm selling RearView (www.rearviewapp.com), and android app which is similar to the Frontback app. It's been online for about 5 months and there are 11.000 registered users and 2800 uploaded photos. I haven't added any sort of monetization strategy (no free/premium version and also no ads). The only cost that it has is a fixed $20 per month of hosting on DigitalOcean.