If they're within $10M or so right now, seems like a deal has to get done. Especially with the way he throws his body around when he plays, the risk of Rondo getting hurt and not getting a good contract offer next summer is real. Rondo's not LeBron; LeBron could lose both legs in a tragic gardening accident and you'd still see a dozen teams offer him a max deal. Short of improvement beyond that what's evident so far, I don't see how Rondo's looking at a helluva lot more $$ next summer than is on the table today.
Still think a shorter extension (like what LeBron signed) makes sense for both sides; Rondo gets a good amount of money locked in, but alos gets a chance to prove himself to be a MAX player (which no one believes he is currently) and become a free agent again in a couple years. Celtics aren't locked in to any overly long-term committments, and the pressure stays on Rondo to keep improving. Three-year, $32M or so, maybe a little less if the third year is a player option.
If Rondo's as good as we all hope, he'll be a $16M - $18M player in three years' time, and Danny won't mind paying him that IF he's proven that.