It is probable that for appearences on established programmes such as 'Never Mind', the fee is set in advancec and offered to everyone regardless of perceived celebrity.
One-off appearences are negotiated by the artist's agent and are 'how long is a piece of string' - because fame and schedules vary so much, so depending how much the artist is wanted, and how much free time they have, will affect the fee upwards or downwards.
Andy's second para is correct even for regular series such as NMTB. The perceived celebrity level of guests is always the deciding factor - but other non--financial factors come into the equation too. So they may be getting similar payment on paper, when other 'favours'/payments are included.
I know what you mean Quizmonkey - I read that, for payment to appear on Space Cadets, William Shatner accepted a week's stay in the producer's cottage in the Scottish highlands.
But no-one has come up with a figure yet. What about �3,000?