@mutterphore, I write literary fiction, and the originality of the plot is less important than other elements like voice, language, characterisation, setting etc. If you write psychological thrillers, mysteries or crime novels, plot originality will be more important. The really good instances will have all of the above, obviously!
I don't think that setting out to write a novel purely for money is likely to work, especially if you want to publish traditionally -- it can be hard to second-guess the market, and what it will be doing once you've have got an agent, and that agent has got your novel in front of an editor who buys it etc is even more difficult to predict.
Obviously, if you're writing subsequent novels, you will sometimes have to choose between retreading familiar ground that your readers like and have come to expect, but that's not what you're asking.
I would write the kind of thing you read and enjoy, whatever that is.