So much depends on your motivation. Business travelers trying to get the last few thousand miles at year end to keep their status might pay more than that.

You have to put a value on what you expect to achieve from the run then compare it to the fare you will pay.

My personal goal is $0.00 per mile because I try to combine them with bump opportunities and I like to stay at the destination for the weekend if possible so many wouldn't call these mileage runs, but since mileage for international trips later is a major motivation I consider these trips sorta mileage runs.

To really understand if a certain price is worthwile you also need to state your motivation for making a mileage run.