the digital SPL meter is about $50 but there is a non-digital one that is about $30 if I remember.

Do they use the same mic and have the same calibration curve?

I don't remember the line-in ever being throught of.

I'm sure it was thought of, but the problem was that you either (a) couldn't sample it, or (b) couldn't sample it while playing digital audio files (don't remember which). I think that what Genixia's talking about doing works around both of those by using a different method that we didn't think of before. I'm not sure I understand his diagrams completely, but it looks really promising.