You're basically asking, "What is the cash value of a feat?"
That's going to change as you level up. Look at the feat you're currently giving up for the crafting feat. How much cash would you pay for an item that gave you that feat? Some feats have a fairly quantifiable cash value - the Dodge feat, for example, is +1 AC, so you could compare it to the price of an Ioun stone that does the same. Other feats seem almost priceless.
But it doesn't seem worth measuring. The disparity between two characters, where one has taken a crafting feat and is using it to benefit the other, is small compared to other forms of disparity.
I played through a campaign as a cleric. I took Craft Wondrous Item and crafted for the other characters without charging them extra. I made it my number one priority to keep everyone alive. I spent my own money on scrolls to take care of any nasty conditions that might afflict them. The question of whether my character fell behind theirs was meaningless, because I rarely did anything in combat beyond buffing and healing them. Towards the end of the campaign they pooled their money and bought me an expensive Metamagic Rod of Quicken Spell. Did I come out ahead? Who knows? I wasn't keeping count.