It is unquestionable that Columbo is very dedicated to his job regardless of the salary. However, his refusal to accept Brimmer's lucrative job offer has more to do with the fact that he suspects Brimmer of the crime. Having suspected Brimmer from the first time they met (as referenced in the last scene of the episode), Columbo knows that the job offer is a set-up to mask the truth--and this is proven when Brimmer tells Columbo that he would not be working on the Kennicutt case if he took the job. Perhaps Brimmer should have been more cagey about that.
I realize that David...and I was going to add that point in...but my post was getting a little "long-in-the-tooth" and I didn't want to ramble on longer than I did already.
Just a great post Cassa. I didn't read your post at work, I saved it to read it leasurely at home. And I am glad I did. Great observations and I agree with every one of them...very insightful stuff.