The amount of investment you should make in SEO depends on a variety of factors, including the percentage of your business you intend to get online, your gross monthly revenue, average customer value or average sales price, competition within your market, any ad spend and time frame for expecting results. Small businesses, generally -speaking, should allocate 7%-12% of their gross revenue to their marketing budget. The amount of your marketing budget you should dedicate to SEO depends on the type of the business as well as what other marketing is being implemented. If you are a web based business and 100% of your sales come from online traffic, SEO could be 100 percent of your marketing budget. If you are a local contractor that provides a local service such as HVAC, you may want a large portion of your budget to be in ranking your Google Business Profile. Local SEO (ranking your Google Business Profile) can be less expensive than traditional SEO which is ranking the pages and blogs on the website. At the end of the day, search engine optimization is designed to provide a ROI for the business.  If your SEO agency can generate $1.50 for every $1 you spend, spend a million dollars with them.