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Hello, It sounds like you are receiving this error because you have the markup for price in the html, but it is not finding the price so it is showing the error. If you remove the schema markup for the price it will still be valid and should no longer show the error. We have a similar setup on a client's site of ours. They offer multiple versions of their product in different versions and we do not markup the price using schema, only the ratings and these show up perfectly fine in the SERPS. You can also check Google's Structured Data Testing Tool to aid in schema implementation. As for a price other then a numerical value, looks like that is possible according to schema.org.

Great insight guys. I knew I would be going with dashes along but I was having a friendly debate within the office. Thanks again!