"Overvalued" means "I don't think it should be worth this much" in this case.

The fair value for cryptocurrency, just as with everything else, is determined by the market(s) (at least, when it is unadulterated by government obstruction and price-fixing and such). So to determine the fair value of a bitcoin, go to the exchange of your choice and look at the price. If you don't like that price, try a different exchange. You might not want to pay that price right now, and that's okay, but that's what the fair value is right now.