With the help of the guide at https://www.invastor.com/blog/165042-How-to-Calculate-Link-Building-ROI-and-Justify-Your-SEO-Investment/, this company offers a way to calculate ROI from link-building. https://www.invastor.com/blog/165042-How-to-Calculate-Link-Building-ROI-and-Justify-Your-SEO-Investment/