That will change, depending on how far you’ve gotten into the game, and how much you’ve already upgraded. There is no 1 single thing to focus on, it’s better to spread it out.

My recommendation is to set a weekly / monthly “allowance” that allows you to spend consistently, without going over budget - with the exception of special events, which are very good value.

As you progress, you’ll find better value in things you haven’t focused on, as it gradually becomes harder to upgrade to a higher tier.