The top paid apps overall might get 10's of thousands of downloads per day, but in general it takes much less than that to get into the top 100 of a category . For example, I had the top paid productivity app about a month ago, and was getting anywhere between 200-1000 downloads/day while it was in that spot. That app also made it to about #65 in the top paid overall. I also had the top paid finance app back in May, and was probably getting between 200-300 downloads/day. That app never made it to the top overall at that point (it had been in the top 50 overall for a while in 2008).
Free apps are a different story of course. On one hand you will get more downloads than a paid app...on the other hand so will all of the rest of the free apps, so it might be harder to break into the top 100 whatever.