I took a look at the code. It gives one card per level until 10, then 15, then 20 and every ten levels after that. The chance of high rarity goes up substantially after 10, so you'll be getting rarity 4's pretty often and sometimes even 5's. Considering the chances of getting a rarity 5 otherwise, the percentages for level ups are damn good. And if you recycle one 5, you'll get enough for two 5-card booster packs, which is 10 random cards... or ten level-ups worth of cards. The 4's yield half that, enough for 5 random cards in the shop. Remember, each level has four chances to get a high rarity card, because you get to pick your choice of four. Also, when you get to the really, really high levels it's a rarity 5 every time (!). Overall, the "absolute value" of the level ups keeps going up.
"Scissors are overpowered. Rock is fine." -Paper