If you want something super cheap, a Clarke Sweetone. If you want something that's more of an intermediate instrument, I'd get a O’Briain Improved, Freeman Tweaked, Dixon Trad or DX005, or Hoover PVC. Those are just recommendations from experience, there are a few threads on here that have lots of recommendations. https://www.reddit.com/r/tinwhistle/comments/1fq77yf/pinned_whistle_maker_list/ https://www.reddit.com/r/tinwhistle/comments/179avhc/a_request_for_a_pinned_thread_of_all_whistle/
Several people here recommend the Dixon DX005 (plastic) as a good mid-price starter whistle in the $50 range.
https://reddit.com/r/tinwhistle/comments/1nsape6/thin_whistle_for_learning/