Put there whatever if you just want to satisfy validation tool and makes you happy.
Check item.php
My brother, with your response " if you just want to satisfy validation tool" it makes it look like we are some weirdos. I work many years on SEO on casinos sector. Since you decide, that when we disable price, you still create the structure of priceoffer and you set it to "0" (which means no price), you still need to define currency - it's just how it is. It's not to satisfy google validator. It's to satisfy the guidelines of GOOGLE, so it doesn't count it as error on your site, so you can position as good as you can on their search results.
Anyway its easily fixed by adding one line on the oc-includes\osclass\scructure-data.php if anyone wonders.
Thank you!