I would suggest you take a look at this https://dotesports.com/the-op/hearthstone/news/hearthstone-guide-stat-budget-explanation-29730 it is an old article and does not have some of the new abilities ( Although since discover adds a card to your hand, then it should be worth 3 stat points just like drawing a card with a battlecry) but it is pretty good at explaining how much stats go into a card and how the abilities affect the mana cost.
I think for a balanced card that would see play, you can make it slightly undercosted for its mana cost but has a twist on its effect so its not broken. A good example is Tar Creeper which is a three cost card that has the stats of a 4 cost card on your opponents turn or Blink Fox which is a 3 mana card with an acceptable statline that instead of drawing you a card from your deck, adds a card to your hand from your opponent's class. Another good example is The Lich King. You can also make a minion have high stats for its cost but comes with a drawback that is not too severe.
Basically as long as the card has acceptable stats for its cost and a good effect that gives it value, then you are good.