It does mention on the OZ page "You'll earn miles based on flight distance and the purchased fare class and/or fare class flown" meaning not on price.if not on a UA ticket.
You did not mention the route and fare class but the uncapped amount would have likely still been well below the ticket price approach.
This is a fundamental difference in earnings from a UA ticket versus a partner ticket. Remember originally partner tickets earned no PQD/PQPs.