Your answer can be found here:

https://sellercentral.amazon.com/seller-forums/discussions/t/3c5ec562-8318-4c3d-92f1-1ef412606e31

ELIGIBILITY AND COST ASSOCIATED

The eligibility criteria outlined below is per country that you sell and publish A+ Content in. You may meet criteria in one country, but not another. You are eligible if:

You have already published A+ Brand Story to all ASINs within your catalog that you own. You can confirm this by accessing the A+ Content Manager and searching to see if you have created and published a project. You will also see Basic A+ Content and A+ Content Brand Story on your ASINs under the “From the Brand” fields on the detail page.

You have had at least 15 project submissions of A+ Content that have been approved within the past 12 months. You can confirm this by accessing the A+ Content Manager and checking the status of your submitted projects and the last modified date. This is to ensure our guidelines are being adhered to. Access to Premium A+ features will be available to 3PBrand Owners at no cost during our promotional period for all ASINs registered to their brand. If we announce a fee for Premium A+ Content, 3P Brand Owners will be given the opportunity to opt-in for a fee. We will not charge any fees without your consent. Currently, we have not determined an end date for the promotional period or a fee structure.