I've read your blog post with great interest before clicking through to your website. Let me tell you, after reading what you wrote, I expected pricing ranges starting from at least $200 (ok, that could be just me, but still - $50 is waaay to cheap). Your customers are people who spent weeks and probably months writing their Thing, and Kindle is probably the hottest publishing platform at the moment - and they are the people with serious problem of converting Word to Kindle. Us geeks, we often forget how it feels not to know what we know, how it feels not to be a geek. In my line of business, I deal with many non-technical people, and they are scared and intimidated by all this technology and software.
What I'm trying to say: What's a $100, $200 one time fee compared to the possibility of making a writer's dream true? It's possible you're leaving serious money on the table and methinks you can get very, very - rich in the Kindle business (I surely hope you do, because I think you provide a very valuable service).
What if you tried experimenting with pricing?