I once looked into something similar to this. I found that on the windows server with classic asp, the raw html of the pages sat in text files and not in the MS SQL database. So what I envisoned was taking the text files and grepping them to remove unneeded markup - like tables, font calls, etc. - and then pasting the text content (some with image links, etc) into wordpress pages.
But that still requires editing all those pages in the WP editor. There's probably a way to write an MySQL query that would generate the pages directly from each text file into the database.
So, is your content in text files or in the database?
Anothger thing: do you need to "translate" asp routines that the exisiting site uses into php?