I am looking into starting up an online classified ads website so I'm
looking for relevant industry data regarding pricing for both online
and printed classifieds.  If possible, I'd like it broken out
nationally and for Houston.

Quick note: this is part of a larger set of questions (parts 1 and 2)
so any help on them would be greatly appreciated!!

Well each site has it's own demographic.  An online classified ad site
that is used predominantly for muscle car ads may have more of a male
bent.  Costs are also determined by the target market.  If you charge
x amount for an ad that reaches 3 people then x has to be pretty low. 
But if it's reaching 100 thousand people you can charge more.

For the houston area I found these that might help you guage your pricing:
http://www.houstontribune.com/class_ad.html
http://houston.bizjournals.com/houston/services/advertising/ratecard.html
http://www.zwire.com/site/news.cfm?brd=1574&pag=460&dept_ID=542578 (a
particularly helpful one I found)
http://www.nationwideclassifieds.com/nation/papers.html (Also
excellent.  Shows comparisons with circulation and cost)

Online pricing varies, and there is a tremendous market for free
classifieds.  Another option may be for the owner can maybe take a
small % of the sale/transfer.

Hope that helps.