Think about it a second.... Ok, now when Yahoo sends you traffic is it most likely from their search and NOT from their directory? If the answer is yes, then you have to ask if their backlink is worth $300? If your answer is no, save your $300. But, the only directory I would even consider paying for is Yahoo and I haven't yet. Too many free ones out there. Remember, Yahoo traffic is from their search engine NOT from their directory.
I can see *some* market for it- not in my area though (to pay for it.)... Depends on what you sell and if the people look through directories. I try to have sites that relate to the products Im selling but are only informational resources, siting my e-commerce site as a supplier. That way you get the PR once removed from the E-commerce site, without having to pay for it.