How can I decide if something that cost a large sum of money is a practical expense?

Something is a practical expense if it saves you money elsewhere, or facilitates your earning potential. If it does neither of those, it is not "practical". Weigh each purchase decision using these factors, especially when you are young, and you will soon build a strong financial foundation that will get you to the point where you can make some "impractical" purchases.

When that "something" saves you a lot of money on other services or goods in the future.

As in buying a good used car with cash is a one time large purchase that saves you a lot of money in the long rung on insurance, bus tickets and the actual chance of pursuing a job that bring in all the money to pay for things.

Wasting your money on the big screen T.V..... is not a practical expense.