>>>If all this volatile value is just selling future... then, know what, anyone who buys something they aren't quite sure how it works without taking economy 101 through 401, they do deserve that their real money goes the way of extinct imaginary money.
>>>
>>You continue confusing things. Company shares are not company assets; it is different. Company assets are accounting term consisting of cash, plant, inventory, equipment etc. Company shares are certificates confirming ownership of these assets.
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>If company A owns a number of shares of company B - how do these shares count in their balance sheet? At what price?

In this particular case company A will have to report these shares as assets, under 'Long Term Investments' in the balance sheet.
There are different methods to evaluate this kind of assets, well regulated, by the way. Most common way, currently, is mark-to-market valuation when company adjusts this asset value on quarterly basis, taking non-cash earning adjustments. All these numbers must be reported to both SEC and individual shareholders. Please, note that mutual ownership of shares is rare, i.e., as a rule, long-term investments comprise minuscule part of total company assets, except some financial companies due to specifics of their business.