According to online averages, the ROI for SEO is 22:1, or $22 for every $1 spent. Generally high margin service companies are above that average, and low margin e-commerce companies are below that average, with e-commerce averaging an SEO return of 2.75:1, or $2.75 for every $1 spent.
Given that ROI is dependent on industry, it is recommended to talk to an expert and get an idea of what your company could expect.