Check the post I wrote above, all is in there what I did. But it's only one strategy out of many so as I suggest trying stuff out yourself to what works best for you.
For me it worked out okay, not amazing. For 10 dollars I would get around 2 subscribers. Sometimes you would gain 5, sometimes none for 10 euro's. Retention would be similar to real viewers, sometimes higher, usually a little bit lower (make sure you don't run kids content on e.g. pewdiepie's channel, the retention would suck). Wouldn't suggest blowing all your cash on one video, try several videos to see which works best for gaining subscribers. Also make sure that your video has end screen and cards optimized so that people click through to your content.
I suggest you check it out, do some tests, check some youtube videos of people using different settings, check earlier posts from
@KiddieToysReview and
@FamilyToyReview, I learned most from them.
Most valuable lesson I learned, blowing cash at bad/not trendy content/thumbnails will not work well. I thought my content was good at the time, but I was not growing at all. Only after I changed my content to something more trendy I started to grow. Also when you analyze your adwords data, you will be able to see the click to impression ratio. If you want to optimize your thumbnail design, you could e.g. run a two day campaign on a video, change the thumbnail and run it for another 2 days with the same settings. Then compare the click vs impression (CTR) and see which thumbnail drove more clicks. That way you can learn wich thumbnail design work better and make your channel better (and get better result from adwords)
