How is the EVE Online currency valued?

You can buy a PLEX, which means Pilot License EXtension. Those can be bought for real money and then can be traded ingame for the virtual currency ISK. On Activation, a PLEX grants 30 days of playtime.

At the moment, 1 PLEX is about 600.000.000 ISK or 20 USD if bought from CCP.

There is some base to this figure, but it is also reported in a very misleading way in many cases.

First, you can buy ISK with real money, and you can calculate an exchange rate between ISK and USD that way. The way this works in practice is that you can buy a PLEX with real money, and you can sell this PLEX to another player for ingame money, effectively converting real money to ingame money.

The most accurate data on the losses in this battle come from a CCP devblog:

The Economic Impact

11 TRILLION ISK.

According to some PLEX conversions that could equate to approximately $300,000-$330,000 USD.

The part of the quote I want to draw your attention to is the use of "could" here. If you would want to buy that much ISK, you could buy it for ~300,000 USD (if the PLEX market can deliver that much at this price), but that doesn't mean that the ships that were lost in the battle were actually paid for in real money.

There are many ways to earn ingame money, and the big alliances that were involved in the game have substantial regular income sources. They own certain moons that produce a significant profit, and they rent out part of their own space to other players in exchange for ISK.

In short, you could buy 11 trillion ISK for around 300,000 USD, but that does not mean that anyone actually paid that much for all the ships that were lost.

The EVE PLEX market sets the USD to ISK rate

It does this through the selling of PLEX for USD

On the market right now there is a thing where you can get 500 PLEX for $20.00 USD. If we do some quick math that comes out to about $0.25 per single PLEX. Now 500 PLEX is worth 1 billion ISK. Therefore if 500 PLEX is worth $20.00 and also $1 billion ISK then $20.00 USD is equal to $1 Billion ISK.

Therefore the exchange rate is equal to 50,000,000 ISK to $1