programs like adsense and kontera are the easy way around and usually the smart choice initially. it is very easy to implement. with these programs you are not getting paid directly by a business. nor is your ad space dedicated just to them.
with paid vendor ads, vendors pay you directly to display their ads. you can arrange one day, week, month, year or however long contracts you want. the best way to gauge the market is by going to sites that offer advertising. usually they have a section indicating how much it is. go to friendster or facebook for example, there is a section that tells you how much it would cost. this is based on size and duration of the ad. the more popular the site the more money it can command. it is all based on traffic.
go to adbrite.com and look at some of the sites using them. usually you can click right on the adbrite logo and get an idea for how much the site is charging. sign up with adbrite - this is an automated way vendors can sign up to have their ads displayed on your site. they pay you through adbrite and it has really worked well for me.
vendor paid ads are a big revenue stream for me. i suggest working on getting traffic and then contacting vendors who might be interested in your clientele/visitors. alternatively, consider putting up adbrite on your website to faciliate the process for you.
good luck and let us know how it goes!
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