Let me give you a scenario that actually happened to me. I'm curious to know how you would have handled it.
I needed gutters for my house, so I called a local company to get a quote. The owner arrived and saw that I was building a trailer. He told me he really needed a new trailer and he needed his old trailer fixed. He told me that my gutters were going to cost $1,500, but he was willing to work out a deal. I would build him a new trailer, fix his old trailer, and put those items against the cost of the gutters. So, I built him a trailer to his exact specifications. Before I started on it, I made sure he knew that the trailer he wanted wasn't going to be cheap, and he said it wasn't an issue. He installed the gutters and two weeks later, I gave him a detailed bill for the materials I used to build the trailer exactly how he wanted and to repair his old trailer. The bill was right at $3,000 which did not include labor. He said it was too much, he only wanted to pay $500. He suggested I keep the $3,000 trailer that I didn't want and I should pay $1,500 for the gutters he installed.
What would have been the ethical thing to do?
In case you're wondering, this was all done on a hand shake and verbal agreement. There was no written contract.
I would only ask if he had set a budget stipulation before you started. To even consider a custom 1 off trailer would be $500 is a real push.