What do you pay for with each?
With PPC you pay Google for each click on your ad, at a price an auction sets. With SEO you pay for the work, in your own hours or an agency's fee, and nothing per visit, because Google doesn't sell places in its free results. Both cost money. Only PPC charges you again for every visitor.
How Google prices a click: your maximum cost per click "is the most you'll typically be charged for a click," and "That final amount you're charged for a click is called your actual CPC." You can set bids by hand or let Google set them. Google's automated strategies, like Maximize conversions or Target CPA, "fully automate your bid strategy based on the CPA, ROAS, or budget target that you set."
Calls matter most for a service business, and Google treats them as clicks. Taps on your call button "cost the same as headline clicks." Turn on call reporting and Google can count calls as conversions, using "Google forwarding numbers," which is how you see which searches led to a call.
What SEO costs depends on the work and the market; our SEO cost guide has the ranges and what each buys. Google's own Local Services Ads charge per lead, not per click, and get their own comparison in local SEO vs Local Services Ads.