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

[Baa]

I run my own website and my google rankings are now on the first page (UK google search) and I get regular views from Germany, Canda, USA, Australia, South Africa, UK(#1) then some more but much less in other countries globally.
I am looking for companies to both advertise on my website (they will send me an image / artwork and a paragraph to put up on a certain page or on the side bar on main page) and I want some of them to send me products to review.
How do I go about this? just email them and offer + hope for the best? when one contacts me back how do I calculate the charges for the advertising? is there a standard? I definitely do not want to overcharge them but also dont want to charge too little as I have worked real hard getting this site to such high rankings.

PS: most of the companies that would be contacted or will contact me back will be in the US

All advice welcome,
thanks