There's some great data out there on marketing channel ROI. This infographic pulls together industry studies and surveys on how marketing channels compare.
Somewhat disingenuous to separate channels like this. Everything is everything now and that means results in one channel are influencd in many ways by other channels. That is an important caveat for the "all or nothing" crowd who may read these results thinking they can reduce or eliminate commitment to social or PPC.
Yeah we wouldn't do that. Your site is MODELED and substantial changes to any part of your current model could take down all the dominoes. That doesn't mean this knowledge isn't important though your results may be different. Knowing what channel is contributing profit and what channel is contributing traffic allows you to invest in the blend. Remember it is the blend that wins so don't go cutting off your nose to spite your website.