That's a nice site. I remember when the entire marketplace was "The History of Rome" several years ago.
Consider ultra narrowcasting. I don't think the British History Podcast guy (who is awesome to listen to BTW and officially VLM endorsed) is going to demand the full $100 to advertise and would surely result in orders of magnitude more than five clicks, for example.
As a user of your service I'd advise that a big problem with podcasts that are essentially volunteer operations is long dry spells. The bulgarian history podcast, which is much more interesting than it sounds, although the intro music makes my kids cry, had a roughly five month dry spell this spring where I unsubscribed in my podcast catcher because I thought the podcast had completely died. Cool, its not dead. Now I should resubscribe although the point of this story is you need to automate or productize or make more efficient that process, not just dumb luck that I run into the guy who runs the site on HN so I click around some of my old favorites and holy cow the other BHP is back, thru pure luck.
There must be some kind of technological magic that could be spun somehow to help volunteers having a dry spell from having their audience melt away. Reminder service on the web or email to let people know dead podcasts have been resurrected? You might even be able to generalize this magic, whatever it is, to let people know about resurrected open source projects or similar.
"Its not dead, its just pining for the fjords".com or something
I don't know if you're taking show suggestions, but I'd like to recommend The Dollop. The Dollop is a bi-weekly* American History Podcast. Every week, Dave Anthony reads a story to his friend, Gareth Reynolds, who has no idea what the topic is going to be about.
It's hysterical, and has a huge following. They do live shows and everything. Just finished up a tour of Australia.