I can agree with Mosby's post in some ways.
My two cents:
If Paradox is going to adopt a pay for PRC (Post Release Content) scheme, my recommendation is that they mix up pay PRC with free PRC. From my experiences doing such a thing is important for preventing a "nickel and dime" perspective, and since some stuff is still available without cost, fans still get the perspective that the game is being supported for everyone, not just those that are interested in paying for additional content. And really, a problem with Day One pay content is that it becomes much more transparent that this content was developed with the rest of the game, so why shouldn't it be included with the rest of the game?
Even if we're just looking at sprites (though I imagine you can do so much more with PRC than just sprites if you wanted to. Various battle scenarios like Mosby suggested is a great idea).
For Day One PRC though, my recommendation would have been to offer up some sprites as free content. This gets people excited about additional content updates. Don't release all of them, but periodic small updates of a few sprites here or there. But then you can do a bit more focused and/or larger updates that have more sprites.
For example:
Day One content update available for free could be something like Tank Sprites for the Soviet Union and Germany. My impression is that these two countries get played the most, so right away it hits a large amount of your fanbase playing the game. Don't release all of them, but give them say a couple of the popular main battle tanks that people would really like to see. Then follow it up with a larger, small cost release that unlocks the rest of the tanks, in addition to more flavour for each side with respect to more tank sprites, more plane sprites, and more naval sprites.
Though I think timing is important. If this larger update came a bit after the release of the game, it'd give the impression that Paradox is continuing to support the title after release, and it makes accepting the cost a lot easier to swallow because the perception is that there continued to be an investment of man hours (which are expensive) after the release of the title.
Just my thoughts though.