Absolutely agree with hopskipjumppaper! If the blog doesn't get at least a few hundred visits a day, if they don't have at least 1,000 subscribers, and if they don't have a specific topic or if their topic has nothing to do with your products, just turn them down.
As a general rule, I've been told that you can only expect, at maximum, 10% of any blog readers to follow up on a post and actually look at your website or products.
And only 10% of those people who look at your product are likely to buy.
That's a whopping 1% of the people who read the original blog post. And even that is a best case scenario. If a blog's only got a couple hundred readers, you would be giving away a free item to get *maybe* 2 sales. And again, that's best case scenario.