$150/month product that saves businesses $4,000/mo (one less programmer). Hard part: Find 70 clients.
Working in industry, we demo products all the time. We pay outrageous amounts of money for an online time tracker which is basically an online spreadsheet. We have considered obtaining systems for managing accounting, supply, etc.
Focusing on our core business, policies, we never get tied up in 2-3 month/year adventures in uncomfortable territory. There is no innovation, just maintenance. We use a 20 year old 4GL language assisted by native Python/C.
I could crank out Android apps in niches, write web content and advertise, build a few cloud based web products (CAN IT EXPORT TO EXCEL!?!?!!!111), and easily live off of 15-20k a month with huge fluctuations (both directions). I choose the simple life of industry.
Not to mention, it is so hard to motivate yourself to go from start to end on a product. Most of that energy was used in my late teens.
Right now, I could be cranking out that turn based strategy Google Glass native app, because native apps with delays in turn taking are going to be the key to Glass (it gets hot!). Instead, I'll just build another fixed width extract with this day. It will run in 1/16th of the time of the original. People will not notice or care until it breaks again. Ad absurdum.
I suppose its worth mentioning, I know a few who pull in 4-6k a month solely on advertising/affiliates. Completely passive, they have months worth of posts automated to go out every 2nd or 4th week. I've pulled off 1-2k using the same method, but I always had a hard time finding microniches that were available for me to dominate. Writing about en pointe ballet, dremels, silver polishing, simms fishing gear etc. is not my ideal goal. It did make money though.