I just sold a couple low priced domains on Sedo and the commission was less than $10 (10% - rounded up to a while dollar amount). The lowest I've done is $65 and the commission was $7. I believe the confusion is because Sedo positions the fix priced/parked commission as a waiver of the minimum, and the minimum commission is $50. And the marketing speak and actual policies that are enforced by their system don't link or or need to allow for changes.
From their FAQ:
All domain transactions are subject to a minimum commission fee, determined by the extension of the domain being sold/purchased. The party responsible for the commission will pay either the minimum fee (outlined below) or 10% of the agreed sales price, whichever is higher.
Minimum commission fee is 50 USD:
.com / .net / .org <domain list cut>
Important: With Sedo’s current “No Minimum Commission Promotion”, the minimum commission fee for domains with the above extensions will be waived if the domain is both verifiably parked with Sedo AND sold at a fixed price. If the domain is not listed with a fixed price when negotiations start, please factor Sedo's minimum commission into your negotiations. The minimum commission for .tel domains cannot be waived (.tel domains cannot be parked).