Two sessions ago, one of the PCs had his armor severely damaged (nearly destroyed, actually) by the corrosive tears wept by the ravening eyes aka eye beasts they encountered.
I do a very simple armor repair cost:
Armor and Weapon Repair
Armor and weapons can be repaired in town.
Armor costs a percentage of the original cost equal to the damage suffered; round up to the next 1% and the next highest $1. For example, a DR 6 plate corselet ($1300) reduced to DR 3 from corrosion damage is at 50%; cost to repair is $1300 x 50% = $650. A DR 3 Fortify +1 ($1000) Lighten 25% ($2000) Fine Giant Spidersilk cloth shirt ($3270) that is reduced to DR 1 is 67% damaged; cost to repair is $6270 x 67% = $4201.
Weapons that have been cleanly broken cost 50% of their original cost; weapons damaged by corrosion use the percentage system above based on their remaining HP.
Casters with a Repair spells can fix these items instead, but the missing bits cost 50% of the above-listed cost in the case of corrosion, rust, etc.
And that's it. I don't require a roll, and I assume magical fixing get done as well. This is cheaper with the Repair spell from a friendly mage, but that person does have to roll. Don't critically fail!
I probably should subtract out enchantment costs, but a) enchantments can be worn down and need repair, canonically, and b) I let the DR of Fortify affect the survival of items so why not the costs?, and c) it's much simpler than re-calculating the cost of each item. I'm sure players won't mind, but it wastes game time we can never get back again. I'd rather have them spend imaginary money I can imagine up more of.