technically, from a materials POV, bronze can be worked to quite a high quality ... the default assumption that bronze is "worse" than iron is simply a major mistake.
Bronze is more expensive because ...
1. Bronze is *harder* to work (technically - getting the amalgam just right is harder than in basic Iron/steel production)
2. Bronze is more frangible than iron (parry an attack with an iron sword ... you still have a sword. Do the same with a bronze sword too often and you need to get a new sword)
A +0 bronze sword compared to a +0 iron sword ... the iron sword wcould be churned out by the dozen by an adequately trained smith; the bronze swoprd would be the work of a master craftsman.
The simple way of modeling this is to say that bronze should be "by default" a -10 material, but the combination of a skilled craftsman & high-quality materials should mean that a "top of thge range" broze sword and a top of the range damascus steel sword should have equivalent bonuses (+20 or +25 max for both). The rarity would be different - the bronze sword would be one in a million; the steel sword would be one in 10 thousand ...
Iron age warriors beat bronze age warriors not because INDIVIDUALLY the weapons were better, but that the iron weapons could be mass produced ...