Personal or Commercial Use
Google Apps are available to use for free when you have a regular Gmail account. If you are designing lessons to use in your own classroom, you do not need a business Gmail account.
However, if you plan to sell your Google lessons (Docs, Slides, or Sheets) you will need to purchase the business suite.
The fee for the business version of Google Suite is $6/mo per person. If you are running your Teachers Pay Teachers business as a side hustle on your own, you only need the $6/mo Basic Plan.
When you sign up for G-Suite it will automatically place you in a $12 plan. As soon as you have finished setting up the account (there are some technicalities involved) you can downgrade to basic.
If you plan on having someone help you with your business and need multiple emails, it will cost an additional $6 per person.