Fixed pricing is a guess. Usually, it's a conservative guess that leaves money on the table when demand is high, or leaves your calendar empty when demand is low.
The Problem with 'The Rate'
Almost every expert has "a rate." But a rate is a static variable in a dynamic world. If you have 10 people fighting for one hour of your time, your "rate" is irrelevant—the market value is whatever the highest bidder is willing to pay.
Auctions as a Feedback Loop
Auctions provide real-time data. If your slots are consistently selling for 5x your floor price, the market is telling you to increase your floor or specialized even further. If they aren't selling, you know exactly where the market ceiling is. This constant feedback loop is the only way to ensure you are never underpaid.