Two ways to bring people to a channel: pay another channel for a post, or earn attention in discussions. They do not replace each other — the economics differ.
Placements: paying for speed
A fixed sum, the post goes out, a wave of subscribers arrives within a day. The cost per subscriber is known afterwards: price divided by arrivals. The upside is predictability and volume. The downside is the price, and that the inflow stops completely once the post has run its course.
Commenting: paying with time
Constant, modest spend — subscription, accounts, proxies, model calls — and a constant, modest inflow. The subscriber usually costs less but arrives over weeks rather than in a day.
The important difference: commenting does not switch off after one publication. While the accounts work, the inflow continues, and the cost per subscriber falls over time as the one-off setup spreads across more volume.
Audience quality
A subscriber from a placement came on the recommendation of a channel they trust. Warmer, but with less attention: they subscribed in a stream. A subscriber from commenting came because your answer looked competent, and has already seen what you write. Colder towards the brand, warmer towards the substance.
Comparing honestly
Compare the cost of a subscriber who is still there a month later. Placements produce a spike and some of it unsubscribes in week one. Engagement matters too: a thousand people who read are worth more to an advertiser than five thousand who do not — and that is where the money difference actually shows up. The mechanics of the inflow are on the channel growth page.