Tesla shares in the market are worth $17 each to Musk - that's what he got for the ones he sold off. Any money changing hands for them after that point doesn't come back to Tesla, so as far as he's concerned they _are_ vastly overpriced.
Shares are worth precisely zero to their holders sold. Until that point they're a piece of paper with a supposed value, but with no backing other than the confidence of the market. Any other point of view is a gambler's wishful thinking.