Tony, the digital SPL meter is about $50 but there is a non-digital one that is about $30 if I remember. I have one of the digital ones sitting here and I also have the line-in running to my center arm-rest.

This is kind of amazing... I remember this being brought up years ago but the reason given was always that the mic input had limited frequency range. I don't remember the line-in ever being throught of.

EDIT: Didn't see that genixia found the other cheaper SPL meter. About "stepping out of the car", wouldn't a human body in the car effect the response? If we can live with it (the tone), wouldn't it be better to stay in the car?