Here is what I would say:
1. Person: "I heard those have problems" or "My friends had problems"
Your response: "Wow, I'm sorry you have bad luck" or "Wow, I'm sorry your friend has bad luck"
2. Person: "Why did you pay so much when you could have bought ___ for less thats faster."
Your response: "They didn't have any that cost more."
3. Person: "How much did you pay for _____."
Your response: Tell them you payed double what you really did
Person: "You got ripped off, you payed way to much."
Your response: "I got a great deal, I wanted the most expensive one they could sell me."
I have never once had anyone come back with anything to defeat those retorts.
When someone asks you or tells you something like they have been doing, they are not the originators of what they tell you. They heard someone else spout it off. They are expecting you to be stupid enough to believe that they are asking for a genuine reason, and stupid enough to give them a defensive answer.
I once had a guy ask me "Why did you pay so much for the Ducati, when there are cheaper more reliable bikes." I looked over to his Mercedes AMG and said "I suppose for very similar reason why you bought the AMG when you could have easily bought a Lexus." His response was a Big Smile and "Fair enough."