Hello,
Someone is interested in advertising on my site, a 468x60 banner on top,
If I get about 300K page views/month,

What is a good amount to ask for?

Are there any standard formulas people use?
Thanks

The price per thousand of page views depends mostly on the type of audience your website has...

Depends

Type of Site,

Visitors ( Most Important )

Advertiser May type of audience .

And This all can Quote You an Clear rate.

http://conversion.7search.com/scripts/advertisertools/keywordsuggestion.aspx

Go there, search for your KWs and you'll have an estimate of what they are paying for ads. Overture and Google pay more than this one, but it helps to have an idea.