The blogosphere is dragging eyeballs and influence away from the mainstream, so advertisers are going to have to learn how to deal with "amateur media". And it's not the same.
First thing to understand: if you want to get the most from a relationship with the blogosphere, you've got to understand it. Obviously. This means reading blogs and, ideally, writing one. You've grown up with magazines and telly; you know intuitively how they work. But you need a crash-course in blog grammar, and writing one is the best way to get it. (If you're not reading lots of blogs, and you want to start, get yourself some kind of RSS reader. It's a lot easier. Google and Wikipedia will tell you what that means.) If you're not reading blogs but you're spending some money there, you might be wiser to invest your money with those nice men who e-mail you about opportunities in Nigeria.
Second thing to recognise: there's not a lot of money in blogging. Very, very few people are getting rich out of it, and those that are tend to be in tiny niches. You might see this as opportunity (pick up lots of blog readers for not much money), but it doesn't work like that, because most bloggers aren't doing it for money, they're doing it for fun, companionship, attention or any of thousands of reasons that people write or talk.
So your regular space-for-cash transaction may not appeal to them. This can lead to a huge disparity between the value that you and your target blogger put on their content. You've probably looked at the audience, done the maths and come up with a reasonable, sensible offer:probably a few hundred quid. They look at the effort, love and time they put in and will most likely see your offer as insultingly small. It's not that either of you is wrong, it's just that you're valuing their media in different ways.
And this is something we're all going to have to get used to, as more and more eyeballs are seduced by amateur media, in whatever form. Maybe you should start a new department - Customer Media Relationship Management - to develop and sustain all these new relationships, for though they might want less cash than the average magazine, they'll demand far more care and attention.