Unless you luck out and find a "good" detector at a garage sale where the seller doesn't have a clue what he has, all you will get for $50 is Junk and wasting $50.
You can do line Rando suggested above, or either buy a good used detector in the lower end range on the "For Sale board" or look for a New ACE or Bounty Hunter......Still most likely going to cost you $150 up.....
If you watch the "for sale board" used starter detectors pop up there in the $100-$150 and up range.....