It totally depends on what your industry is and what your ROI is (Obviously).
Recently, I have been spending $16 a click on a website I used Google website optimizer to optimize the landing page for a 20%+ conversion ratio.
It's an affiliate site selling a product for $299. I get $200 of that.
You do the math.
I have only recently gotten into affiliate marketing. I mush prefer low maintenance residual and passive revenue, but this is just too good to pass up. 2007 is the year of the affiliate for me. I'm going to pour my heart and soul into it.