How Much !!!!!!

I use a white lens cap which fits onto the Lee filter ring.  The lens caps cost: £3.20 for three - err that's about $1.50 each I think . OK I know you have to be using the Lee filter system to use these lens caps. Just seems that the system in the link has a bit too much added value for my liking, a version of the Lee lens cap is all that is needed and costs would not be more than $2. 
http://www.speedgraphic.co.uk/prod.asp?i=12222&1=Lee+Adaptor+Ring+Caps+%283%29

Yes the idea does work, it is a lot more convenient and practical than carrying a grey card.

So there yer go Bruce get on the phone to some far east factory and get them to knock out millions of white plastic lens caps, in various sizes, and your fortune is made  

Pile 'em high and sell 'em cheap - packs of three assorted sizes for $10 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html