The problem with trying to give named weapon/armor untyped abilities a static number is, they often don't follow the magic item guidelines. So, they don't work out to what you'd expect.
The design team decided the price was balanced for the effects gained at the point the item would be available at that price. This is part of the reason magic item creation is sometimes more an art than a science. You as the GM know what your players are capable of and where the campaign will lead, so you need to decide on the prices in such cases. The guidelines give a starting point but everyones home game will be different and your prices may be considered high to some, then low to others. This is part of the reason PFS doesn't allow named items to be 'upgraded' or altered at all. Items are built at price points and giving those 'undefined' abilities to other items could be unbalancing.
Long story short, do what lets your game progress and allows for the players (and yourself) to enjoy it. Honestly at 16th level 'balance' starts to become VERY subjective, and as a 'martial' character I'd let the item go at its price (not more expensive/costly). Spellcasters are going to be getting access to endgame spells the next level and a continuous custom item is well within the characters budget (if the casters aren't already offering to help the party out by providing the buff themselves for free/using a higher level wand that the character bought to provide it/etc).
As for the feat, if you think the characters should have to sell their items to buy new items and basically be 'penalized' for switching weapons or not using weapons they find (which often times happens with exotic weapons/builds) I guess you could considered broken.
Really it is a way to simplify the characters life and book keeping, they use X weapon and find Y weapon which is an slight upgrade. They either have to choose to sell the weapon (and not get its full 'worth') because it isn't the right weapon for the feats they have, sell the first weapon and sit on the gold because it isn't enough to actually upgrade the new weapon, or some such nonsense.
With the feat the new weapon allows them to upgrade the weapon they are trained with and allow them to 'use' the new weapon more efficiently. It's a quality of life option really.
It's been priced as a +1 enhancement. 73,300-1,300=72,000, which is a +6 weapon.
It's been priced as a +1 enhancement. 73,300-1,300=72,000, which is a +6 weapon.
Or the price difference happens to be the difference between a +5 and +6 weapon. 22000 gp.
@FALCN120:
PFS doesn't really allow crafting by the PCs from what posters in other threads I've read have said (feats restricted). They need to 'buy' the items they want and altered named items are not possible. So it isn't strictly a PFS rule that they are disallowed. Custom items are strictly the domain of the GM, RAW, so do it if you want them to be allowed. I'd price it at the 22k difference as you figured out and account for it as a static cost special ability, it would increase the total price of the item by that amount not as an enhancement cost.