I'm surprised at the negative reaction you've had. I think the mystery is a draw—it's like posting a word search!
As for your question, YANBU but I am not surprised at the questions. Anything unusual will pique people's interest, and I am sure that the disabled and the alien will get far worse than you. Even when it's based on fascination with the unfamiliar, unwanted question can feel like an intrusion.
There are so many responses to "how much did it cost?" that would leave you and the asker feeling comfortable without you revealing anything. "There are more expensive things" for instance, or "Less than I expected actually". Turn it around with "What do you think it's worth?", or compare their own spending if you can think fast enough and feel especially angry!
Like everyone else, your "it's something very noticeable" puzzles me, and unless you have had a third arm attached I think you must mean that it's noticeable to people who have seen you before the operation. I'm thinking along the lines of the removal of a birthmark or something else unwanted.
Anyway Haunted, I think you're clearly enjoying your "modification" and I hope it continues that way. How about spending a few minutes thinking of non-committal answers to the questions that you don't like, and remember that you're probably just as inquisitive as they are about unusual lifestyles!