Unfortunately, it's more complicated than that. If you open a random shopping page, you will notice that price tags are all over the place. Even if the page itself is for a dedicated product, it often comes in various shapes, sizes or flavors, but above all, you always have the related/recommended products and whatnot.
I suppose the best bet to get this done would be to analyse shopping software and plugins. A ton of sites are Wordpress based, and the number of popular templates is probably limited. If you check them out and see which tags or form fields are used where, it's maybe not so hard to figure out the price on any give page.
Since Google are both lazy and powerful, I'm also wondering if they didn't put some rules out there ("follow these rules if you want us to extract your prices"). If such rules exist, they can, of course, be exploited by anyone.
Btw. my hunch it's better to miss a few pages than to have too many false positives.