By default, the Stack Upgrade for Sophisticated Backpacks costs 8 of the mineral for its tier, with the exception of Netherite since it uses the Smithing Table by default. This means that a Tier 1 Stack Upgrade costs 1 Upgrade Base and 8 Iron Ingots, a T2 requires 8 Gold and a T1, a T3 requires 8 Diamonds and a T2, and a T4 requires 1 Netherite and a T3.

The current state of the recipes in the official ATM7 server (and possibly the entire modpack) is 9x cost for T1 through T3, but T4 costs 72x the price since instead of it using 1 Netherite Block in a Smithing Table it costs 8 Netherite Blocks on the Crafting Table. In my opinion, the cost of the T4 Stacking Upgrade is maximum overkill. I see no reason to require 8 Netherite Blocks. Sure, it makes it more mid- to late-game, but I'd prefer something more close to vanilla, with every recipe for stack upgrades being just 9x value.

Those recipes are actually default Sophisticated Backpacks recipes which I try to balance around vanilla. and the reason T4 is so expensive is that you can combine the other tiers together to get pretty high multiplier and if you really need to go as high as netherite at that point you have insane amounts of items for vanilla (I mean 5 diamond stack upgrades will get you 2mil per slot) and you should be able to get 8 blocks of netherite as well.