I think I "don't get this."
Does Garfield mean "If 'money can't buy happiness,' then do you, Jon, rent happiness? Because you are clearly a happy man." Or does Garfield mean "If money can't buy happiness, then do humans in general rent happiness, because I see an obvious link between money and happiness."
Either way, I am not sure why Garfield is acting like he's never heard this platitude before, and even less sure if the panic in his eyes is the proper reaction.
I like to think Jon gave Ellen this speech with no prompting but his own insecurity. This strip also raises questions about Jon's income level and professional success as a cartoonist which are better left un-pondered. Believe me. I've pondered them.