Ask them how much their data is worth. If they lost all of it, could they still do business? No business, even fast food joints or retail shops, can afford not to do backups. Imagine how much work it would be to start over and put everything back into digital format. That's how I get businesses to buy stuff they think they can't afford when I'm doing IT consulting, by showing them that they can't afford not to do it. There are also opportunity costs, where you can show them that inefficiencies cost time, and therefore money, so technology expenditures are a must to be competitive.