Whatever you charge, be sure to treat it as an advertisement and not just a paid link.

The advertisement should be for human traffic only, and Google outright says that ads should have the rel=nofollow attribute to the <a> tag, or else a robots.txt file blocking spiders from following the link.

The bad news for the person wanting to advertise on your site is that no PR would be passed along to them, unless they already know this. But because they're specifically wanting to purchase a 'text' link, it sounds like they're wanting some ranking juice. (which Google is clamping down on from being bought or sold)

But better to have a disgruntled advertiser than to put your own site at stake.

Google's position on Paid Links

Best of luck!

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