After finishing my first playthrough of The Last of Us (hard difficulty), I found that I had accrued $4500 in money that could be used for bonus content (i.e. skins, render modes, art). How did I get this? And I how do I get more? Are there specific challenges to complete? Is the money linked to the trophies?
The money is gained by completing challenges. You should be able to find them under Bonuses -> Tags in the menu system. You collect money (amount of which varies with challenge it seems) for every challenge completed. Some of the challenges are associated with trophies, but the money you earn from that is not linked to any trophies, neither are the unlockables that you purchase with the money you've earned.