How much is my desk worth on the market? However much somebody is willing to pay for it.

As the article states, Apple's "value" as a company comes from, in part, the fact that it's a good investment. Some may argue that "the fundamentals" of the balance sheets and whatnot doesn't justify a $1T valuation, but they would be missing the intangible parts of the business which cause it to be worth exactly that much.