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There are a few ways to keep your listings live on MSN but as far as I know, you will have to pay for each referral with any of these ways.
The first is to advertise through MSN directly as a featured site. MSN has a pretty high monthly minimum for spend and won't even consider your site unless your terms get the searches meet this minimum.
Sponsored listings pull from Overture.com. You'll pay Overture for each of these clicks and you have to stay within the top sites to get listed on MSN.
Web Directory pull partially from LookSmart. Through a deal with LookSmart, you can pay them to push your website to MSN all the time. You'll pay per click for all referrals for this.
I hope this answers your question and helps.
The cheaper option would be to go through LookSmart and pay for constant listing in the web directory category.