First off; LOVE the mod, great idea, simple execution, and lets you keep using weapons you like, that would otherwise be outclassed by other weapons. But, there are some issues. Namely; this mod makes anything upgradeable,
ridiculously overpowered. Like, I can kill eye of cthulu in seconds, with a +15 enchanted sword, which only costs 10 platinum to make, before I'm even done with pre-hardmode.. Money farms are not hard to set up, and allow me to get upwards of 10 platinum in about an hour, which is more than what's needed to get the enchanted sword to +15, and it all-but invalidates all other weapons in pre hardmode, and even some in hardmode. I chose the enchanted sword because you can get it before you do anything else, and sure, it'd be harder to upgrade it without a wire money farm, so it's not that great until you get past skeletron, but that's besides the point. So, my personal recommendations to you, that I think would help with balance majorly are:
1.) Increase the price or lower the value of each upgrade; Either make it so that it costs a LOT to have a weapon that good, maybe a 4x increase in price, or make it so that the upgrades are... Less so. Something like 4x less powerful? The +10 version of a weapon should be the +40 version. 150% (roughly) bonus damage is MASSIVE, maybe 200% bonus damage at +40 if you really wanna be crazy with it, but +1000% more damage just, kind of breaks the game? Like, at +40, an enchanted sword can easily carry me all the way through to golem. Easily, without a doubt. It's stronger than the terra blade.
2.) Limit the amount of upgrades available based on certain achievements throughout the game, i.e; you can only go past +3 once you've beaten the Eye of Cthulu, and +5 once you've beaten the eater of worlds, and +8 once you've beaten queen bee, and +10 once you've beaten skeletron, etc, etc, etc. This way, you can't just, say, leave your computer on with an AFK farm while you go to work/school, come home to 200 platinum, and immediately have a wooden broadsword capable of defeating the moon lord (Mite be an exaggeration but I haven't tested it), plus, basing it on only the vanilla bosses makes sure that it has compatibility with mods, as the modded bosses wouldn't have any effect on the level of upgrade available.
3.) Change the upgrades based on the items, and include tools! Make it so swords get a different upgrade from bows, which get a different upgrade from magic, which get a different upgrade from pickaxes, etc. Tool upgrades wouldn't upgrade the damage, but instead could upgrade the tool's proficiency (i.e, a +0.5 to pickaxe power for every level of upgrade) and speed of the tool, while a boomerang's upgrades would be damage and knockback, or a yoyo's upgrades would negatively influence knockback, making it easier to keep enemies in a damage loop, or making the yoyo stay out longer, or anything like that.
These are just my ideas for this mod though, because I really want to see this mod succeed, as it would quickly get a place on my "must have" mods list, up there with magic storage, veinminer, boss checklist, and other extremely helpful but not gamebreaking utility mods. Hopefully this helps and gives you some insight on the balance of the mod! I'll attach pictures of what a +40 version of a 24 damage modded weapon ended up looking like (Yes, I know, "its modded so of course it's overpowered," thats why I used the enchanted sword as an example for the rest of this "review," this was just the weapon that made me realize how overpowered this mod currently is.)
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