Work this customer up to it, and get him to let you use his name as a satisfied customer.
Charge him halfway to profit on the next sign and let him know you need to go all the way after that.
If customers understand you're losing money on a job, they're much more understanding. Your biggest issue with them is whether someone else is willing to undercut what you have to charge to get to profit.
But I would definitely try to treat your first customers well, even if you lose a little here and there, and provided they are willing to speak up for you. You can't beat customer testimonials to help you sell your service.
Cheers,
BW
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