Usually I go with whatever default price the ROM has unless there is reason to change it. If I do change them I like to set Items to their PoR prices (cause I love that game).
With items that are exclusive to my hack either A). They’re rare and you can’t purchase them or B). I set the price by gut feeling. This approach works pretty well for me because i have an innate sense of game flow and design.
After all that Item prices get tweaked during testing.
It depends on how much money the player will have at the moment of buying and how many copies of that weapon you want the player to have. If the weapon’s really good, make it expensive so they don’t break your game, but not so much they never bother to buy it, ever (looking at you, spear from FE8).
For breakable weapons, you have it easier. Check the price of a similar weapon in vanilla and give it a similar price, then playtest from there. For unbreakable weapons, it’s harder. I personally use multiples of 1000 and adjust according to playtesting.