Google Search Ads are usually the most expensive on a CPC basis out of the big platforms, but usually the CPA is much lower (even though Bing Ads can often be better value). This is usually because of 2 reasons:

1. You can target a specific part of the funnel (informational -> purchase intent) in search ads. Targeting on social networks is more about overall user profiles rather than their immediate state of mind.

2. People going to a search engine expect to leave that search engine to go to another website. Whereas people on a social network expect to stay in that portal. So clicking on an ad then doing something after is a more natural flow (and better value for advertisers).