The two female co-hosts are not doing the same job for the BBC that he is, nor are they as educated.  Ergo the 3 co-hosts all have different compensation levels. 



(The male co-host also happens to be on the UK version of Dragon's Den, is a professional economist with a Masters degree from Harvard and also provides policy analysis for all of the various BBC shows.  Neither female co-host has the same level of responsibilities, experience or education.  

What is surprising is just how much the various radio hosts are paid - £600,000-£1,000,000 year)