Ah "grunge"... I have played with a few "grunge" or "dirt" elements in attempts to weather a skin but they never seem quite right. Staining on aircraft usually follows panel lines or are exhaust streaks or oil streaks from leaks. I do like the brown staining you did. I work in Illustrator when I do skins. I assume you do this in Photoshop since it was a jpeg?
Yes, i know those are not the normal scratch for a plane, i used the first texture i found becasuse i am at the office so i can "play" to much
. Generaly i use photoshop, but for this fast work i used gimp because i am on ubuntu here at the office. But it can be ilustrator for keep it in a vectorial format.
Details like the smoke from exhaust, scratch form pilots steps, old paint marks, some other scratch and tones diferences on the color (like the brown i used) etc... give to the skin an extra look.
I can-t help you with money because
of this. but i am happy to help with ideas and graphic designs works.
You are doing an amazing job with this skins. Wish you good luck.