I daresay he feels that he's worth $600k, but to me that's top tier for a center... and by 'he' I mean his manager who wants a payday.
By low balling they probably offered him $350k, which is probably too far under. I think he's still on his first real deal outside of NYC, so he's probably on $250-$300k and definitely deserves an upgrade.
I think this instance is where player options are useful ... say a $400-$450 with player options for two years.
If he kills it then he earns his right to upgrade to a better contract, but $400k isn't too bad for the broncos to hang onto him.
Instead he'll probably get $600k with two years of player options, just so we have to sweat every 12 months