Ok so here is the brief explanation of commonplace guns
Commonplace Guns: While still expensive and tricky to wield, early firearms are readily available. Instead of requiring the Exotic Weapon Proficiency feat, all firearms are martial weapons. Early firearms and their ammunition cost 25% of the amounts listed in this book, but advanced firearms and their ammunition are still rare and cost the full price to purchase or craft.
Now however, if you have the Gunsmithing feat or are a gunslinger you can make ammo for 10% of the cost it normally would take.
I think normally that is like 2 gold and 2 silver a round.
But with this new things saying their ammo costs 25% of the amount listed.. so I calculate the costs down to 25% then use 10% of that new number?
Yes. The 25% cost is what they cost in the setting. It's not even a modifier, that's the new base number so everything that affects the base number affects that.