Good cpc for google ads - what is it?

There isn't one number. A cpc of a few dollars is cheap in legal or insurance and expensive for a low priced ecommerce product. Work backwards instead. If 1 in 50 clicks converts and a customer is worth $100 profit to you, you can pay up to $2 a click before you lose money. That's your ceiling. Whatever the industry averages say, that maths is what decides whether your CPC is good.