The point is, if 500 million isk is worth 10$ and if it's taxable then every eve player should pay taxes while he play but of course we know this is not true
No. It does not matter how the gamer is compensated, namely isk or BTC, or CAD, etc., but rather is the gamer a professional (taxable) or not (not taxable)? Here is an article on the case of poker players and the taxation of poker winnings:http://taxdood.com/2011/03/07/taxation-of-gambling-winnings-in-canada/ What matters here is whether the player is a professional or not. What the pot is denominated in is totally irrelevant.
By the way CCP can add "Income Tax Auditor" to the EVE Sandbox