Everyday it seems some new method of doing WB on our digital camera's hits the market. Now in addition to Expodisc and the Black/Gray/White Photovision target, and a host of gray cards, and patches, etc., we have the Lally Cap. Here is the information on it:
For this you pay $32 after shipping. Now let's see, that's $32 for a little elastic and a piece of white cloth --- hmmmmm. But Expodisc is even worse as it sells for $100 plus and the cost of production is about $2.00. By the way you can buy the Chinese version of the Expodisc for as little as 99 cents on eBay (search for white balance), but you do have to pay $5 shipping --- makes one wonder.
I've found that a white coffee filter works as well as either of the above and costs a few cents. I use the filter with a rubber band and get results that seem to be almost identical to the Expodisc.
By the way, Expodisc now has the new Expodisc Paddle that only costs $199. Wow --- what a deal! Here is the link about it: