While we can play the market value game, I think Rondo's value is higher for the C's than it is for other teams. Maybe he does look better because he plays with the Big Three (though he certainly dominated last year without KG and with PP and RA worn out). But does it really matter? If he leaves next year, the C's will have essentially flushed the last couple years of the Big Three's time together down the drain.
What he can do for the team in the next 2-3 years outweighs any salary burden he may be 4-5 years from now.