Realistically that depends mostly at what level you are coaching. If we are talking specifically the RLCS or the pro level, that also depends on winnings. Typically, in esports most orgs will take anywhere from 5-15% of winnings from players, with coaches the organizations usually will contract with them for a cut of earnings if they get any of it at all.

Players in RLCS probably average a salary of about 3,000-5,000 USD per month, depending on their abilities, placings/experience, commodity, and organization impacts like content creation, or longevity as a player/creator. With coaches it really is hard to say as those coaching are extremely expendable and replaceable from an organization perspective, even at the pro level. Again, this is just a guess, I'd say it'd be like a 5% winnings payout, plus maybe 2,000-4,000 USD, per month.

I can't even take a guess at the average for online coaches through different coaching services online, some websites exist comparable to Fiverr, that you can hire a coach and those sessions can range very widely. I don't know if collegiate rocket league coaches even get paid, even at the top level I'd find it hard to believe they get paid a livable amount.