Board Thread:Game Discussion/@comment-212.143.231.38-20130611092427/@comment-5706972-20130612170831

To answer the original question:  Cost to upgrade (which is based on the stats and rarity of the sacrificial card) and rarity/level of card being upgraded make a difference. When the card is level 1 you get the most experience increase, and level up more quickly than at level 49 (or 64 for R4)

To answer the most recent question:  I am relatively new, but I have seen 0 stat increase from leveling up - all it does is recharge your 3 regenerating cubes and energy, give you a (usually bronze) medal and (sometimes) increase your ally capacity.