In real terms, 10,000 gold can be bought for 35 Euro. The real life equivalence of 400,000 xp is very hard to express, as the amount of xp you get in one battle can vary widely from as low as 100 xp to as high as 10,000 xp. But for the latter you not only need to win a game with a great personal performance, you also need a bunch of special circumstances like xp multiplier bonuses from events and missions. If you play a regular tank repeatedly, the amount of xp you get per battle is, order of magnitude, somewhere around 1,000. So 400,000 xp is about 400 battles, a battle is about 5 minutes on average, so this is 2000 minutes, or nearly 35 hours. In other words, I exchanged 35 Euros for 35 hours of my time.

Of course that is not quite correct, as I want to play 35 hours of World of Tanks anyway. But the way I use my free xp is never to research any tanks with it (researching a top tier tank is around 200,000 xp), but rather to use it to research all the most important modules on a new tank. In other words, instead of playing tanks that are underpowered because they aren't fully equipped for 35 hours, I paid 35 Euros to play those tanks at full strength for 35 hours.

You can call that Pay2Win. On the other hand World of Tanks is an old game, with a bad treatment and thus retention of new players, so most of the players you meet are veterans of the game. With my 12k games, I still count as a noob. Players that have been playing for much longer than me not only have their tanks fully equipped, they also tend to play the more powerful tanks, while I am handicapped in having to play the tanks that get me up the tech tree. If I would be playing stock tanks with stock modules instead of spending free xp to get the better modules, I would just further gimp myself against an opposition which has all the advantages. It is the MMORPG equivalent of trying to play PvP with a character that just hit the level cap against other players which have been at the level cap for a long time and not only know have played a lot more PvP than you, but are also decked out in full epic gear. World of Tanks is basically a World of Warcraft in which the level cap is still the same as in vanilla, and there were never any stat/gear resets.

If somebody made an identical copy of World of Tanks, and attracted an equal number of players, but everybody had to start afresh, that curiously would be a much better game. It is the curse of a progression-based game, that without resets the progress accumulates to a point where veterans are not just more skillful, but also have huge stat/gear advantages over newer players. But of course as World of Tanks is in the business of selling progress in the game, they can't do a reset. They are stuck with a game in which the veteran population destroys their chance to attract new players. Something went very wrong with that concept.