A few years ago I started transferring all of my old audio tapes into MP3 files for my player and PC. Really easy and simple to do... and of course CHEAP!
I have a stereo player I plug into my microphone (stereo) input on my PC. My player has auto-reverse, so it will continuously play the entire tape, both sides, automatically.
Next, all I have to do is start up the free sound editing application "Audacity" (http://audacity.sourceforge.net/) and begin recording off the player. The nice part is that I can capture the entire tape at one time, then later on go in and easily cut out each individual song and save them as individual MP3 files. The quality is pretty good - better than 'good enough', and I then got rid of all my old Mary's Danish and REM tapes on Ebay. About 5 minutes of effort per cassette.