Hello,
The changes you suggested where already like this in our shop.
Now I have found out, that the problem lies with this part: currency=EUR in the link google uses to crawl:
?currency=EUR&utm_medium=cpc&utm_source=google&utm_campaign=Google%20Shopping (I dont know if I am allowed to post our site 
When I do Rich Test from Google with the normal link on my page, the microdata shows the correct price (gross). But as soon as the part currency=EUR is added, the micro data/rich test shows only net prices.
Does anyone maybe know where this could result from, as we cannot make google to remove that part. Our default currency in the shop is EUR, might be something with the schema?
Thank you for all the help!