what is a reasonable charge for advertising on website/s?

[Open Sauce]

It doesn't usually work like that, you would normally run a pay perclick prgramme of some sort, like google adsense: https://accounts.google.com/ServiceLogin...e&hl=en_US
Although adsense pays very little for sites with little traffic, if you're getting a fair amount of traffic, you'll get a better profit. You could also sign up to affilliate programmes for a higher profit.
You would need to find products or services related to, but not in competition with your business, then check that providers website to see if they run an affilliate programme (often listed at the very bottom of a site), then see how much they pay per sale. A good web hosting company for example, pays 50 dollars per sale.