I've said this before, but you can't warrantee a product from nature. I've never offered anything in wood to my clients, especially when there are better alternatives. The price of the substrate shouldn't be in the equation when pricing a sign that you expect to last 5+ years. It's minimal between MDO and say, 6mm ACM. Most ACM has a 20 year outdoor durability rating. Why wouldn't you use it? If they don't want to pay, give them a banner. It will last as long as a wood sign. Since they probably think their business is going to be temporary, give them a temporary solution.
Just flat out ask them, "So you're telling me that you only expect to be in business less than 2 years?" Store Owner: "Uhh, well... no, of course not." You: "Then why not get a sign that will last you many years without having to replace it? I can tell you this [MDO/Wood Product Sign/Whatever Else Cheap] sign, will only last you maybe 2 years before it starts to deteriorate."
We are oftentimes only dealing with a difference of $85 - $100 on the high end. Seriously, are you pricing your work so close to the margins that $85 will break you or make you lose the bid with the business owner? If so, that's not a business you really want to deal with if they don't understand lasting value. Or just offer to give them a free replacement banner sign after a year.