Well I have all 3 MM, AFM, MB and may be a little bias here because I have owned my MM for about 12 years. My personal opinion as to why the price is so high is because people do not sell them. Sure they come available from time to time but you just do not see them very often. I was offered $10k for mine 7 years ago and turned them down.
Also, I would not say because you had a good game or two that the game is extremely easy. Have you battled for the Kingdom (have you beat the game)? I beat a TOTAN the second game I played. I have not come close since. I have beat MM twice since I have had it (Ya I know I am not very good), but for me the humor and trying to beat battle for the kingdom is what sucks me in.
Last point is you said that TAF is a much harder game. For me it is probably the easiest game I own. I have toured the mansion multiple times.
Finally, to each their own. This is why it is great we have so many options for games. Fortunately for me not many people agree with your opinion otherwise the game would be $4000