First, let's deal with the costly part of the compared services. Both contextual advertising and SEO provide, relatively speaking, 2 areas of costs: the cost of the work of specialists and the marketing budget.
With the work of specialists, everything is more or less clear: in SEO, the client pays for the work of an optimizer, copywriter, programmer; in contextual advertising, for the work of a PPC specialist, marketer, analyst. Usually, the cost of SEO work is more expensive than the cost of contextual advertising. This is due solely to the amount of time that specialists need to complete the work: it takes, say, 80 man-hours per month to optimize a site, and 40 to work with contextual advertising. Hence the difference in the final cost of work.
Now about the marketing budget. In the case of contextual advertising, everything is extremely simple: the entire marketing budget is spent on buying clicks. The cost is more difficult. Plus, this is superimposed on the inertia of search engine optimization. Even if you stop investing after 10 months of SEO work, the traffic persists for a long time, gradually decreasing, although there was no work or financial investment.