What do you think about me not paying for my friends car wash after I threw up on her car because I already cleaned it off.
Did you offer to give them money for the car wash ?
Yes of course, but then I found out that the car wash service is too expensive for me
It can be surprising when you find out something is more expensive than you expected, but you did already agree to pay for the car wash. You are the one who vomited in her car, so you really should follow through on getting a thorough cleaning for your friend. If you back out they may lose trust in you.
I don't think they'll lose trust in me just because I didn't pay
Your friend will lose trust in you because you will appear unreliable if you back out. Just explain that the cost is more than you expected, but offer what you can afford.