It is all based on what you do to your site just as the other people have said on it. Enough said... haha. I own and operate over three dozen sites on the internet. And all of them very much different ratios. One is 80/20 another 55/45 and another is 35/65. I have done things differently with each and everyone of them. I think it all depends on what you do. If you can try and get as much referral/organic traffic as you can. You never know when the SEO formula will change again. Plus it is a whole lot cheaper.