They want to reuse the board but all of the info on it will change every year? If they want *you* to peel off the old stuff, it would be cheaper for them to get new ones every year. Give them 1 year of peeling off the old sponsors and they will be happy to pay for a new one. Coro or banner is the way to go. The price isn't the substrate - it's the time involved for layout, printing or even more so, cutting, weeding, taping and applying.
It's the classic "we want a cheap sign, and really durable, that we can use every year and make it say whatever we want for little or no money". Well, I want a cheap car that I never have to put gas in or maintain and I want it to last forever.