Estimate on cover/screen for chimney?

Twice now our cat has discovered a squirrel inside our fireplace. (Propped front door open, and opened fireplace screen..it ran outside both times) I know they make screens/covers for chimneys, but I was wondering if anyone had a ballpark figure on how much it would cost. I'm looking for answers from a chimney company. I could do it myself, but don't have a ladder that would reach, and I'm afraid of heights! lol