600 because it's a building that is immediately able to recruit elite units, Variags cost 550 because they are on par with the Gondorian "Anorien Soldiers" in terms of stats. And in order to get the Anorien Soldiers you need to build a barracks and upgrade it to Rank 2 which cost 600.
Their unit is fully upgradeable, has a boosting power unlocked after a certain rank and they can shred a lot of the cheaper units and hold their own against units of the same tier i.e the Anoriens and Gundabad troops.
I'd suggest to lower the Easterling rax cost to sth like 450 and Variags to 500 so that Mordor has at least one more starting build order. Atm you can only start with a massive Orc spam and have to stay on that for quite a while until you have enough stability to add Trolls/Variags/Siege or whatever.
Unless you specifically want Mordor's earlygame to be purely Orcs ofc.