At the moment, I don’t know you, your business, your website, your resources, your competition or your product. Even with all that knowledge, calculating ROI is extremely difficult because ultimately Google decides on who ranks where in its results – sometimes that’s ranking better sites, and sometimes (often) it is ranking sites breaking the rules above yours.
As of 2009, there are only a few large markets where Google is not the leading search engine. In most cases, when Google is not leading in a given market, it is lagging behind a local player. The most notable example markets are China, Japan, South Korea, Russia and the Czech Republic where respectively Baidu, Yahoo! Japan, Naver, Yandex and Seznam are market leaders.
All sites have a home or "root" page, which is usually the most frequented page on the site and the starting place of navigation for many visitors. Unless your site has only a handful of pages, you should think about how visitors will go from a general page (your root page) to a page containing more specific content. Do you have enough pages around a specific topic area that it would make sense to create a page describing these related pages (for example, root page -> related topic listing -> specific topic)? Do you have hundreds of different products that need to be classified under multiple category and subcategory pages?
The errors in technical SEO are often not obvious, and therefore one of the most popular. Mistakes in robots.txt and 404 pages, pagination and canonical URLs, hreflang tags and 301 redirects, http vs https and www vs non www versions: each of them can seriously spoil all efforts to promote the site. One quality SEO website analysis is enough to solve all the main problems in this part forever.
Onsite, consider linking to your other pages by linking to pages within main content text. I usually only do this when it is relevant – often, I’ll link to relevant pages when the keyword is in the title elements of both pages. I don’t go in for auto-generating links at all. Google has penalised sites for using particular auto link plugins, for instance, so I avoid them.
At Yoast, we practice what we call ‘holistic SEO‘. This means that your primary goal should be to build and maintain the best possible website. Don’t try to fool Google, but use a sustainable long-term strategy. Ranking will come automatically if your website is of extremely high quality. Google wants to get its users to the right place, as its mission is to index all the world’s online information and make it universally accessible and useful.