800k just to hold one more weapon? That's a robbery.
4x4 pages and 10 pages for weapons and 10 for masks in total.
20 5x5 pages for weapons (to make grouping & comparing easier); 20 4x4 pages for masks.
You can increase or decrease amount of columns, rows and pages using Slots.lua file.
Buying new slots costs just the same as it is in vanilla game.
I hope you enjoy it. Unzip More Inventory Slots folder and place it into the mods folder inside your PAYDAY 2 directory.