A way to judge a placement before you pay, and a number that stops you.
Newsletter and paid directory placements are the only channels here that can be evaluated before you buy. Do that evaluation.
Ask two past sponsors what they got per thousand subscribers. That number is the only one that predicts your result.
A newsletter that will not tell you who sponsored recently is not one you should buy from.
What result makes this worth repeating, and what result means you stop. Decide it before any money moves.
Queue-skipping upgrades rarely return the fee. The exceptions are the handful of directories that genuinely send traffic, which you identified in the backlinks guide.