In terms of metrics, and benchmarks, how do you evaluate the relative effectiveness of your SEO marketing campaigns?

SEO marketing campaign is slightly an oxymoron. If you mean to evaluate the effectivity of your search engine optimisation (SEO) efforts, calculate E-commerce or Goal Value from Organic traffic in Google Analytics minus your cost (labor or agency).

 If you mean SEM (search engine marketing)/PPC campaigns (AdWords, etc), again your profit should be your key metric so calculate Revenues from AdWords campaign minus AdWords expenses (aka monetary ROI).

You will probably need an external tool apart of Google Analytics/Google AdWords interfaces to do that. If you want to avoid manual number crunching in Excel, consider using Free Dashboards and Calculations for Google Analytics and AdWords (I am co-founder) and deliver it in automated way.

Thank you for your invite. I second the answer posted by Mr. Zaid Mahadin as I am not technically qualified to give a formula for evaluation of SEO performance. From my practical knowledge and experience of application of digital marketing into our business, the performance of SEO is measured through evaluation of Key data points, Collection diagnostics data, recording website KPIs. The analytic metrics, baseline and benchmark evaluation arithmetically is perhaps as advised by Mr. Zaid Mahadin, so please follow his advise. 

I agree with the experts answers. Thank you.