While you can use the below trick to save money and upgrade items, the 30 upgrades may need to be done to pop:
However, the behavior of this seems to be different from the 100 Weapon tally and you should be safe to do this trick to conserve
To be better safe than sorry, simply grind out junk weapons by going back and forth on the map using the Expedition feature. On the top-right of the map (Japan and inland continent shoreline) almost all the random encounter enemies will drop weapons that have open perk slots.
If you need more spare Enhancement (upgrade) items, you can buy them in the maxed out Town shop menus.
A trick to get the achievement and not waste any Gold (money) and not waste any enhancement items or weapons!
This should be attempted near or after the end of the main Story Missions and preferably maxed out all the towns so you have ample amount of weapon enhancement items,
Go to the 3rd town that starts with a K, on the top right of the map. On the menu of the town is Bolta's Smithy.
Now just upgrade weapons over and over by mashing the A button until the achievement pops.
DO NOT leave the menu or it might Auto Save!
Now DASHBOARD out of the game! When you return, all your Gold and items will still be there.
You can use this Gold to buy expensive Foods for hard fights/second playthrough.