This is his last shot at making money. According to washingtoncapitaslfriendly, his career earnings have been about 40M, but you have to assume with all the stuff Chris Pronger complains about on Twitter (taxes, escrow, etc), he has earned about half of that.
20M is still a lot of money, but if the Senators offer is 5M-10M more, that's a huge chunk of money compared to what he has made so far.
I agree with you about cup chasing, but I think this could be a situation where money ultimately trumps that because we're not talking about a player with 100M+ of career earnings.