Monté is about to start making $8-9 million per year. Phoenix would have had to not sign one of its other mid-level guys to accommodate him. He would probably be at least as good of an option as Cameron Payne as a secondary PG and fit well with that lineup going forward, but he'd be pricier right now. I would not be sure it would be worth not having, say, Jae Crowder right now in order to have Monté on the Suns.
Haliburton is cheaper, younger, and has a higher ceiling than Monté in the NBA, as much as I love both of them. I would think that Haliburton as a (healthy) Sun right now would at least getting the minutes at PG when Paul was off the court and probably a few more, too, given how good he is off the ball.
Yeah, Phoenix screwed that one up. A long-term replacement for CP3 is something they're missing right now.