I doubt the Jazz want to pay him 18 million a year. If I had to guess they were offering more like 12-14 a year. I wouldn't give Favors 18. Bigs are not easy to trade right now, even good ones.
Mh, I think Favors versatility to play both big positions, commitment to smart play, defense, switch and extend his range/skill level make him worth at least 18. If he's more or less healthy next year that's what I expect him to command at least in free agency.