The conditions under which gatekeepers provide online advertising services to business users, including both advertisers and publishers, are often non-transparent and opaque. This opacity is partly linked to the practices of a few platforms, but is also due to the sheer complexity of modern day programmatic advertising. That sector is considered to have become less transparent after the introduction of new privacy legislation. This often leads to a lack of information and knowledge for advertisers and publishers about the conditions of the online advertising services they purchase and undermines their ability to switch between undertakings providing online advertising services. Furthermore, the costs of online advertising services under these conditions are likely to be higher than they would be in a fairer, more transparent and contestable platform environment. Those higher costs are likely to be reflected in the prices that end users pay for many daily products and services relying on the use of online advertising services. Transparency obligations should therefore require gatekeepers to provide advertisers and publishers to whom they supply online advertising services, when requested, with free of charge information that allows both sides to understand the price paid for each of the different online advertising services provided as part of the relevant advertising value chain.