Hi WMCA,
Unfortunately, I doubt that the community can provide a resource that says 'if you improve local seo, your traffic will increase by x%' - you're assumption that these numbers are highly dependent upon the locale and website is correct.
You can do something like the following:
- Identify key, local terms that your company will target
- Determine the overall local search volume for the above terms
- Use search term ranking estimates for each term (i.e. come up with a SERP rank for each local term)
- Estimate the CTR for each term (use your historic CTRs with estimated improvement)
- Use the above to conclude to the total new traffic your site will get if all criteria is met
- Use an estimated ecommerce conversion rate (again, based of historical data if possible) and AOV to calculate new revenue earned