When you spend money, or expend money, you're using money to buy things, and an expense is something that you buy, or something you have to pay money for. And something expensivecosts a lot of money.
We call things expensive when they cost more money than we think they should. If you buy a small bottle of water from a vending machine, and it costs $5, you might think, "Eek! That's so expensive!" If you go to a restaurant, hoping to spend just $10 on lunch, but then every lunch at the restaurant costs at least $50, you'll probably think, "Sheesh! These lunches are so expensive!"