How about I pay value price for a car from my parents even though I really owe them?
Sounds like a plan but why do you think you owe them a value price on the car?
I don't want them to waste their money
It's good to feel the need to want to pay back your parents, and paying a fair price for the car is admirable. But sometimes a good parent realizes that they didn't have a child to be paid back financially, rather to be supportive and appreciative of their kid.
I can give my parents $500,000 in the future.
You are a good person to consider your parents' financial gains, but parents will usually want the best for their child. Like you said, work hard and make a ton of money so you can give them a lot of money or buy them nice things for them.