I doubt the member you know that earned $1200 per month was a sole case by being on a customised deal. If I recall correctly, for a short period at one stage all the Chipmixer participants were getting paid more than $1200 per month because the price of Bitcoin was high enough to take them over $300 per week.
You recall correctly! But it wasn't because the Bitcoin price was high enough, it was guaranteed over $300 per week if someone made the allowed 50 posts to be paid as it was a flat rate for every participant of $6 per post. Depending on the month, it was ~ $1300-1350 per week.
In many places of the world, an account in that signature campaign would get them above average income per month.
That is one of the beautiful things about the forum ecosystem because of the way it has contributed to the benefit of such a large number of members. The economy it has generated (by virtue of signature campaigns, buying/selling goods, services, competitions/games, loans and crypto exchanging) has flourished.The imbalance is there for us all to see when posting in a forum as a Chipmixer participant back then would earn a minimum of $1200 per month when (as you rightly stated) it is above average income for many people around the world. That figure will drop significantly from new years day, that is when we will be able to estimate a median campaign rate.