You do not get change. And be sure to note the proper terminology and attributes of cards.
The white triangle = runes = money to buy stuff
Red circle = power = strength to kill monsters in the CR (Center Row)
The star = honor = victory points that win you the game. The plastic gems that come with the game (cardboard tokens in the Apprentice Edition) are also honor added as part of your total score
The top right of the card shows the card's cost to purchase in runes, or to defeat with power (vs. Monsters).
The bottom left (star) shows how much honor you get at the end of the game.
When playing cards that give you runes (like Mystics), they add to your total # of runes to spend. You do NOT get handed out plastic rune or coin tokens when playing them. You keep track of them "in your head". If you played 3 Mystics, that's $6 (or runes). You purchase a $3 card, now you have $3 left. You may then purchase another $3 card. You now have $0 left, so you end your turn.