Martin was quoted as saying “well, EX-CUUUUUUUSE ME!!”
I don’t know much about art collection— is “buy high and sell low” the general rule?
He’s known for being a shrewd and tasteful collector; perhaps Campendonks have been trending low? 
Apparently, Martin is a serial victim of fraud:
Martin is well known to be passionate about art and it’s been speculated, both here and elsewhere, that sometimes he would agree to do a movie simply so he could use the proceeds to further his appreciable collection, the point being that his performance appeared devoid of any real enthusiasm or ‘altruistic’ effort.
In other words, this 2004 purchase may be the result of 2003’s Cheaper by the Dozen.
Well, it’s only fair then.
Something I don’t understand about the art world is why the signature is so important. Is any part of the price related to the quality of the work?
There are some forgeries that are valuable in their own right. The famous Vermeer forger Hans Van Meegeren, who defrauded many Nazis, is highly collectible.
During and after the Renaissance when there was no such thing as quality color reproduction art collecting nobles would send their best court artists to copy, with permission of the owner, works by Rembrandt and da Vinci and other great names, and some of these copied the signature as well. Rembrandt, eternally cash poor due to lavish lifestyle and a huge entourage, was very p.o.d that some people knowingly paid more for copies of his work than he made for the original. Sometimes these centuries old reproductions get passed off, sometimes honestly, as the real deal.
It’s not just the signature, it’s the creativity, the mystique, the fandom, the bragging rights, all sorts of reasons. You may get some artists who can forge a Mona Lisa, but Leonardo came up with the original, the one which inspires talk about who she was, questions about why the enigmatic smile, etc. Everyone else doing a Mona Lisa is just figuring out how to reproduce his brushstrokes. That takes some talent, but not near what the original did.
The forgers who really impress me are the ones who learn to paint in the style of a particular artist, and create style-perfect, period-accurate “undiscovered works” offered quietly to interested buyers. Obviously these are few and far between, and take a lot more ability (and shrewdness) to produce.
Hell, they should have confiscated a couple of the genuine paintings he already had.