mind your wallet!

 To me, it means "Watch your wallet" so no one steals it. Keep track of it in a crowded place. Keep it close to you.

or

You have $10 in your wallet.

You want:

-to go see a movie at a movie theater

-buy that cool T-shirt

-meet friends for iced coffee at Starbucks

-get ice cream

Is $10 enough for all of that? No.

Minding your wallet means establishing a budget and deciding what's important and what's not. It also means deciding what can wait and what can't.