Can anyone advise ... I need to be able to advise a client what they should charge a hotel chain for banner ads on my clients' site. Does anyone have an idea or can you direct me to a resource for the answer? Thanks!
There's really no set price for such a thing. It really depends on the site's traffic, where and how often the ads will appear.
As an example Google's ad network can deliver highly targeted ads to folks, with a high success rate, and some folks will pay $4-$5 per click for those.
A site with millions of visitors a month can command $100K/year for ad placement and a full-page dedicated spread in a convenient location.
One small company I'm aware of is trying to charge $25K for home page banner ads for a site with 1000 members...and they're still looking.
If your client's site is targeted to a small local environment, check local phonebook and newspaper ad rates to get a ballpark, and adjust from there. If it's a brand new web site with no visitors, you're better off providing the service for free for six months, or else the hotel chain is likely to drop the ads very quickly after realizing they're getting no ROI. If you weave them in well enough, you might get some credibility by associating with said hotel.
If you'd rather not bother with setting prices (or at least feel more in control), recommend a decent low-barrier-to-entry ad network like Ad Brite, in which you can set a price per click and let the hotel chain manage their metrics. (I don't work for Ad Brite, but have used them in the past, with some success).
Good luck =]