I made https://www.myothernumber.com (online temporary sms/mms/voice numbers) about a 1.5 years ago, was making ramen money within 3 months.
Now tracking in the 5-figure/year range.
It's a very, very crowded market so primary cost is user acquisition, but then again that's exactly why I built it, to better learn (consumer facing UA) and have a platform to experiment with.
About to officially launch http://artistic.af (neural artistic transfer meets instagram + canvas printing). I expect this one to take a bit longer to scale, but it was just an excuse to learn DNNs.