Just for a stretch, since you will have $25, $50, and $100, maybe a $75.
I know when I buy a $100 prepaid with my Canadian money, I can never actually buy the Mythic pack, since it doesn't cover it all.
A $75 pack would enable people to buy that plus 2 $10 packs. Not only that, but it also may just be a good idea overall to hit all 4 quarters
Other than that, right now premium is a good deal for what is being paid for. As the game expands, I know we will see changes and more packs roll out. But with current game progression, I think this hits well.