Depends what market you are comparing with. The breakfast show presenters on Capital Radio, for example, appear to earn substantially more than the BBC paid Mills despite the fact there are three of them and their audience is less than half that Mills was getting on Radio 2. Amanda Holden reportedly gets about £3M per year from Heart Radio for being a co-presenter on their breakfast show, which also has a significantly smaller audience than Mills was getting.
As another poster points out, most people at the BBC don't earn the kind of huge salaries that a few presenters and journalists get. The BBC employs over 21,000 people. Fewer than 70 of their presenters and journalists earn more than £178k and most, if not all, of these high earners would get paid even more if they went to commercial stations.