The digital Radio Shack meter is $60, the analog version (which is more accurate, since you can read fractions of a dB) is only $30 or $35. I have it for calibrating my home system and it works great.

You also don't have to modify it for good readings. Search the web and you can find corrections based on frequency (it's about 10 dB low at 20Hz and 1.5 dB low at 60 Hz).

I saw one of those bass competitions once (did not compete, my stereo is stock). One guy in a CRX blew out his windshield seal and a van had four guys holding the doors shut while the guy took readings. If they didn't the bass would literally blow the doors open! Crazy stuff.

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I used to be very hardcore into spl competition, and it is fun to go to local sponsored bass-offs. They are much more fun than db drag racing. Don't get me wrong though db drag is very popular and it is fun, but it is way too extreme these days. The only way you can really get to finals and win a the comps is you have to spend(in my case) every penny you have buying audio equipment, your system will not sound very good at all, and you will be adding about 600-700 pounds to your car(if not more).