That depends on the traffic your site generates, whether those visits are targeted specifically to your site's content, whether the affiliate program matches your site's content, whether those visitors will be inclined to click/use the affiliate service, etc. There are a lot of variables that can effect the actual income your site generates.

I imagine that if you had a book published and it was available through Amazon, your site's visitors (provided they were looking for your book, or something relevant to your site's content) might be inclined to purchase it through that company. That's one situation where I think Amazon would work nicely.

Some folks pull in several Pounds/Dollars per month, some only .10-.30 - depending on the variables above. And then there are the very few who've managed to retire early and vacation in Hawaii

Current stats _ response to person in UK with 1,500 hits per day:
http://www.webmasterworld.com/forum20/1871.htm

UK/US Forum discussing various affiliates (click-through/purchase commission):
http://www.webmasterworld.com/forum20/

Essay on affiliate programs - are they worth it?
http://pandecta.com/affiliate-programs.html

-ms tabby