If you're promoting your own channel on YouTube is not going to be as easy as you think. Besides to your channel brought in the income we need to see to it that he was always in the top, and it requires a lot of time and money. It may happen that you will spend more than earn.
So why should he even pay for the channel promotion?
Focusing on the content released, on your fanbase and new videos will bring you more profits (not money this time) than promoting it, because you will be thay guy, who is doing a great job, but is also underrated.
When your videos have already good quality, the content you have is actually interesting or useful for others, then promoting should be a next step to achieving success.
What promoting a shitty channel can give you after all, when people dont even want to watch the videos made by you?
Nothing, which gives us a very important rule:
Content first.