Commodity: noun, plural commodities. 1.an article of trade or commerce, especially a product as distinguished from a service. 2.something of use, advantage, or value. 3.Stock Exchange. any unprocessed or partially processed good, as grain, fruits, and vegetables, or precious metals. 4.Obsolete. a quantity of goods.

So if you're saying its a commodity (and therefore should be taxed), it has value (and has to be based on something).

But semantics aside, taxation is theft, even more so in this case - there's no justifiable reason to hand over a portion of the gains to a government that had no involvement in the creation of the source of revenue.