Does anyone have any idea of what I should be expected to be paid for writing for a comedian? e.g. is it a percentage or fixed amount per joke?

Do you mean a well-known stand up comedian?... If you are lucky enough to get through to one who will give you the light of day... I think it does vary, I have heard sometimes you don't get paid you might just get your name "out there".

I think generally they get friends who are comedians to write for them, the only way I can think of writing for a comedian is for a tv show, when they do an opening monologue.

Its a relatively unknown comedian but he is known to producers and wants jokes for standup and material to develop TV shows.

if he's unknown then there is no rules of payment really. but I'm no expert on the issue I'm sure Slagg or Lewis will know more than me.