If you really want low cost, then how about:
1. The sound card - you've probably already got it and you
can do A/D and D/A conversions with it as well as using the
game port for a few bits of digital input.
2. Printer port - Again a few bits of digital I/O
Depending on what you want to achieve, you maybe able to
do it without buying any cards for the PC.
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