I've made it mechanically advantageous to hold on to mundane treasures, and it works especially well with jewelry. I allow the value of an item to offset the cost of converting it into a magic item. That 1,000 gp necklace lowers the market cost of a periapt of wisdom by 1000 gp. Find loot of significant value and you may only need to spend some time and xp. I see it as setting up a similar efficiency with wanted vs unwanted magic items. If you keep that magic sword, you keep its full value, or you can sell it for half. With mundane items, you can capture that gp value through item creation or convert it into half that value in coins.