As you said, storage is not likely to be the most expensive part of your database anyway, so claiming "Google's BigTable Costs 10 Times Less than Amazon's SimpleDB" is misleading at best.
Fact: Structured data storage on BigTable costs 10 times less than structured data storage in SimpleDB.
Those two statements are not the same. Implying they are is very disingenuous.
"But the comparison of pricing for two services is not a claim"
Using one of four different factors (completely ignoring S3, which in and of itself isn't exactly fair) to compare pricing is making a claim that isn't supported by the evidence. If you provided a typical actual breakdown of storage costs versus access costs, and then used the total differences in price structure to arrive at a comparison price, then could make a claim about the comparison between the pricing for both services that was actually based on facts.