That is a fairly subjective question because it could depend on where you fly.

Many FlyerTalk members consider their threshold for a good “mileage run” to be 3¢ per Medallion Qualification Mile or less, so one might probably pay $30.00 at the most for 1,000 Medallion Qualification Miles.

Then again, your threshold may increase depending on the cost of flights departing from your own home airport that earn Medallion Qualification Miles. If the best you can do, for example, is 5¢ per Medallion Qualification Mile, then perhaps you might settle for paying 4¢ instead of 3¢ per Medallion Qualification Mile if you were to buy them outright.

This does not take into account Medallion Qualification Miles earned from credit cards, but I am certain someone else can address that in greater detail than I...