How much of the blog(s) are you dubbing? Is there a time cutoff? I could only imagine using this on a regular basis for a daily commute, and occaisional for long travel trips.
So, as an example, I have a 45 min commute either way, so I can listen to up to 1.5 hours of blog posting. Let's assume 1 min per short post, or 5-10 mins per long post. So I could listen to up to 9-10 long posts or up to 90 short posts.
Also, there are other factors such as adverts, voice tone, etc. If you are reading an advert in between each short post and it all sounds like Stephen Hawking, then I don't think I'll listen to many of those posts at all. If I'm listening to advert-free material, maybe I could see myself paying for a rate like iTunes; 0.99$ for 15-20 minute chunks?
The other problem with this is that many blog posts are like news; their value will deteriorate over time. So, up-to-date blog posts will be worth the 0.99$, but day-old or week-old posts will be worth less.
Just my two cents, T.