
Sounds like the wrong way round!
The thing about a calibrated grey card is that you can use it for both exposure and white balance. The inside of a lot of camera bags is grey, often quite neutral grey which is the important thing, but none of the manufactuers advertises them as such - I have three Lowepro bags and they are all very slightly different and none of them is exactly 18% grey (they're lighter).
The reality of white balance is that it's not an exact science and colour consistency is usually more important than absolute accuracy, though sometimes not even that matters much. You wouldn't want a sunset neutralised! What
looks right usually
is right unless you're shooting for a clothing catalogue or something mega critical.
I use the white balance presets mostly, or if not then anything white that's to hand - ceiling, door, car, tablecloth, back of an envelope. Sure there's room for error there, but it's usually pretty damn close. These expensive white balance accessories certainly work, but seem to be a expensive sledge hammer to crack a nut. I've got a calibrated card in my bag but I can't remember when I last used it!