The way I look at it is that the 15,800 avios option means that you are buying 24,700 avios for $320, i.e. you are paying 1.3cents per avios.

This is against extracting value of 3.8c per avios on the base price you quoted. Using that comparison, it is worth paying for the avios at that rate - but only if you think you could better or equal the 1.3c per avios in the future.