The Radio Shack meter is not accurate, but the correction values are known:
10hz.....+20db
12.5hz...+16.5db
16hz.....+11.5db
20hz.....+7.5db
25hz.....+5db
31.5hz...+3db
40hz.....+2.5db
50hz.....+1.5db
63hz.....+1.5db
80hz.....+1.5db
100hz....+2db
125hz....+.5db
160hz....-.5db
200hz....-.5db
250hz....+.5db
315hz....-.5db
400hz....0db
500hz....-.5db
630hz....0db
800hz....0db
1k.......0db
1.25k....0db
1.6k.....-.5db
2k.......-1.5db
2.5k.....-1.5db
3.15k....-1.5db
4k.......-2db
5k.......-2db
6.3k.....-2db
8k.......-2db
10k......-1db
12.5k....+.5db
16k......0db
20k......+1db
Yes, that's a pain in the butt to do, but if you want to calibrate your system for SQ, it helps a lot. I don't want to be in the car when it's hitting harder than 140db, because I like my hearing intact. I pegged my RS meter at 126db doing a 15hz tone, which is equal to about 138db at the least. That was plenty loud, and when I'm listening at normal volumes, it never really gets over about 120db (corrected).
So if you want a cheap db meter, the RS meter would work great for normal listening, and if you want something that'll look nice when you do crank it up, it's always fun to see the needle pegged //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif. Unless you have a pure SPL system, in which case, the Epic 150 is probably your best bet. Sure, it's only good to 150db, but how often do you really want to be going louder than that while in the car?