I designed a custom publish key in wordpress that takes extra values like $cost, for instance. I'm able to save them and recall them within the back-finish, but how do i display them on my small new single-publish.php page?

At http://codex.wordpress.org/Post_Types seems:

As the only-type-template. In the same manner that posts are proven by themselves page with single.php, custom publish types will use single-.php whether it's available.

So for that above example, you can produce a single-acme_product.php file and also the product posts could be proven by using their template.

Which means that you may create a brand new single template file using the title individuals new custom type and display there the particular data for custom type, example:

single-publish_with_cost.php