Things are worth whatever someone is willing to pay for them. Putting aside your opinion of the rapper, if someone is willing to pay $20M for his house, what do you care? And why shouldn't he get that if he can?
There's no right or wrong in this, there's no price judge, if no one's willing to pay it, then the price will have to go down. I don't see a problem and yes, I think it's realistic if it really happened.
If you had bought that house for $3.5M and someone offered you $20M for it, are you seriously claiming you'd turn it down on some moral grounds? Take the money and donate it to charity if you feel guilty but people can pay whatever they want. Incentivize builders to put up more subsidized housing units, you can't blame them for wanting to optimize profits.
If you're arguing about income inequality or zoning variances, that's something different, but how else do you explain Starbucks? Or bitcoin? Or McClaren cars?
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