This is too wide to answer.
Firstly there is a massive difference between B2C and B2B.
And between fields - fashion, celebrity etc.
Anyone who thinks it is a simple impressions per euro calculation is sadly naive.
Secondly there is a long tail effect here. Some companies have their head stuck in the sand and believe they still have the pulling power they did in the eighties so their prices are unsustainably high. This is particularly prevalent in some of the countries that are also slow to move into this century. We have a two-speed Europe and it shows very strongly in media.
And thirdly ads themselves are changing. There is a strong move to native advertising (what used to be product placement), advertorial and sponsored events, moving away from traditional ads. Ads are often linked to in-store promotions (indeed much advertising spend is not aimed at buyers, but at chain stores to persuade them to stock and promote products), online engagement and cross-media.