So to address question 1 - and disclaimer - I do represent CodeNgo here, it is definitely worth it compared to the alternatives (= ad spend). The organic installs you will get will be 1000 times cheaper than any CPI campaign anywhere. You can typically expect 20% uplift on your downloads, but it all depends on the app. That being said, it does not mean that you don’t need to spend on CPI if you want serious numbers. CodeNgo can actually help with that too at rates typically a lot cheaper than Facebook or other sources (advertising is currently in private beta).
As for free, we don’t know of any. Obviously you have the mega publishers like Mobilestreams or Mondia Media, but they will take ownership of your app and typically command a rev share - and will want you to put in their monetization capabilities.
In the end the value prop is simple - do +30 stores yourself and spend X hours, or spend the $20 and 10 minutes.