First of all, pay attention to how much profit the card brings per hour and how much it costs. For example, a card costs 10,000 coins and brings in +950 profit per hour, so this card will pay for itself in about 10 hours, so you can evaluate each card and decide on upgrading it. Also take into account the fact that each next level will be more expensive, but it will also bring more profit; here also use the rule from the previous paragraph.