This will get you started thinking in the right direction. Don’t know where you are, but college towns have consignment and second hand stores. Goodwill and Salvation Army Stores for trail clothes, maybe some gear.
REI Garage Sales can be a gear gold mine. Or a complete waste of time. Get there real early.

Quote Originally Posted by Venchka View Post
Buy the backpack LAST.
When you have EVERYTHING else on hand shop for a backpack.
Walmart, Amazon, Academy Sports, etc. will have suitable 50 liter - 70 liter packs new and available for fitting. Usually under $100. Sometimes WAY less.
WhiteBlaze For Sale occasionally has the suitable older ugly duckling pack in your price range.
Thrift Stores, college students, Craig’s list, etc. are places to look. Garage sales too.
Good luck!
Wayne
Wayne