You'll probably get different answers on this one, but personally I don't believe it's worth paying for, but to people who don't know any better it might mean something.
All it means is that your code isn't vulnerable to the injection attacks they test for and that your web host has a firewall, which most do. Personally I'm more worried about what an online store does with my information themselves than a hacker targeting them to try to steal it. :)
If anyone has any experiences of these increasing online sales, I'd love to hear about it, I just don't see it happening. The way your site looks will do more for your sales than any badges.