Ad spaces is bought and sold through virtual auctions, and as in all auctions, prices are dependant on demand. The more advertisers who are interested in one particular impression, the higher the cost will be. Having a sufficient budget is important to be able to win auctions for ad spaces with higher demand.
We are using CPM (Cost Per Mille = Cost Per Thousand Impressions), which means that we are trying to buy the best possible inventory for you, in order to reach the best audience for your campaign. An AI is working to find that audience and bid the highest quality impressions at the lowest possible price, but also determine where your budget will be most effective.