With FBReader and a few hacks, you should be able to read everything *almost* natively on the iLiad.
It can natively read PDF, html, txt, and prc.
FBreader should allow it to read rtf (use Word to convert your .doc to rtf) and iSilo.
You can also setup a shell script that'll automatically convert your .lit files (using covertlit) into a format that's viewable in FBReader as well.
Edit: I just noticed your price limit. Looks like the iLiad may be a bit too much for you.