In a recent article by Matt McGee, 102 retailers were surveyed about what percentage of their sales were attributed directly to their SEM (search engine marketing) campaigns.  According to the survey, 27% of people who responded to the survey said that search engine marketing (which includes SEO and PPC) accounted for more than 50% of online sales.

Please see entire article here – Search Marketing Remains Strong For Retailers: Survey.

Although this article focuses on SEM, which is a combination of SEO and PPC (paid results), it should really give you an idea of what your ROI can be compared to other traditional forms of media by investing your marketing budget into Internet marketing.