I don't know what it would cost, But why would you powdercoat something that will get scratched?
Let me voice my opinion,
$5.99 for a can of Rustoleum (any color) for each bumper with leftover paint to use for touch up.
Save your money for other parts
Bumpers are just that "BUMPERS". It is what it's called, It's there to "bump" things and it will get scratched up, no matter how careful you are.
How do you touch up powdercoat? You can't!
You can seal the peeling powdercoat but that's short term. It will come back.
But then again, this just my opinion.....