Absolutely not true, and for a number of reasons.
First reason is that nobody is privy to the negotiation that took place. While you feel (might even feel certain) that perhaps the Mavs might've been able to get it done for less, you don't really know that at all. You don't really know if the Knicks have already been approached by other teams about Chandler, you just don't know that. What we're all definitely certain about, is that this is the package that allowed Dallas to get it done.
And the second reason is that if Chandlers turns out to be THAT guy for the Mavs again, did they really overpay? What was his value when he walked off from Dallas as a champ? Was he worth more than a decent PG, a project and a couple of picks? yeah, he was worth more than that. All these trades are a gamble. A leap of faith. In due time, all these deals always look different. RJ now looks like a terrible pickup and doubling down on extending him looks completely stupid. When the Kawhi/Hill trade was made, the consensus was that it was an even trade. Most people don't buy that anymore. Getting Diaw looks like a steal now, but if he stays fat and unmotivated, we would probably be singing a different tune.