I think I’d rather it be cumulatively more. Like the more affix levels, the more it uses or the wider the range gets.
This is how it works. The average is around 1-15, but it varies based on tier for affix shards, and is also different for different runes. For example upgrading to tier 2 costs 1-10 forging potential and upgrading to tier 5 costs 1-22. Keep in mind that these exact values are subject to change as we’re not completely finished tuning the system.