The going rate I expect.
But I agree with Fuzzybee - I've heard too that he donates money from certain work to charity. I guess he earns and has earned so much that he doesn't need it all.
Did you watch the programme on TV about Timothy Spall sailinga round Britain? Well he did this over a couple of years but stopped off in post every now and then to do a few voice overs. One of them which ran throughout that time was his advert for Wickes. It obviously paid for the trip and is good money if you're well known enough to command the fee.