You should be proud of the grind...wait...what?!?! Thanks to Blue's for the heads up on this one.
The intent is to provide players with a sense of pride and accomplishment for unlocking different heroes. As for cost, we selected initial values based upon data from the Open Beta and other adjustments made to milestone rewards before launch. Among other things, we're looking at average per-player credit earn rates on a daily basis, and we'll be making constant adjustments to ensure that players have challenges that are compelling, rewarding, and of course attainable via gameplay.