Session musicians aren't guessing. There's a number, but not one number.
Picture an artist booking a string quartet for a new record. They call four trusted musicians, describe it as 'just a quick session', and ask what they charge. All four quote the same number, not because they colluded, but because they all looked at the same thing: the Musicians' Union recommended session rate.
The MU publishes a different recommended minimum for each type of work, and the logic behind the ladder is simple: the rate tracks how far will travel and how hard it is to undo. A quiet studio album date is the baseline. A live TV slot is broadcast straight to a national audience the moment it's played, so it sits above the baseline. And some jobs (library and especially) are agreed as a flat buyout: one fee that clears every future use upfront, because the hirer is buying certainty rather than a single morning's playing.
This episode is about what those numbers are, where they come from, and why the difference between a buyout and a session-with-residuals is the difference between a day's pay and a quiet income for years.