It all depends. Is it your job and you didn't do a survey or was this a job done for another company who didn't want to pay for the survey. In either case
whoever was responsible for the survey is at fault. My main income stream is from
sign installation (mostly electric and a LOT of channel letters) for national accounts and I always cringe when a customer doesn't want to pay for a survey. I word my quotes "price based on information in the drawings provided". However no contract wording covers everything.
I'm guessing that the main problem is running the wires. Assuming it's LED you can snake the wires around it in 90% of the cases thought it may entail some spelunking. Tapcon the letters to the wall instead of pin-mounting them if that's the problem. Yes it takes more time and you have to work that out with the customer.
Once in a great while (maybe a dozen time in the past 35+ years) there really is a hidden beam that you can't see in a survey. This happens mostly when a property owner saves money when remodeling a property and just puts a storefront on top of the existing facade. In that case you go to the customer and tell them that there is a hidden beam and the price quote is no longer valid. Most people are reasonable and you can work something out.
Take the long term approach look at it. Will you make more money by splitting the cost with these people and doing more with them in the future or do you never expect to work with them again? Only you can answer that question.