You're looking at the price the wrong way. That estimate means that "two-way" could be, maybe, 225€. The car service costs are the SAME for both one-way and round-trip. There are not hordes of people waiting in Positano to taxi to Salerno. In fact, they probably want to go to Sorrento, and that puts the Salerno driver even farther away from his base and home. And it's quite far, with heavy traffic delaying the driver who could go to another job if it exists. Also, Positano is a high-end destination, so prices are set higher.
We haven't had a post on this lately (Summer 2023), but the ferry dock construction in Positano is reported to be behind schedule. You should check very carefully about the "real" ferry schedule, and if they are going at all.
Now, it doesn't apply to your direction of travel as much, but our host, Rick, has mentioned in some of his books that if you are sleeping in Sorrento and seeing Positano and Amalfi on the same day, you should see Positano first. In the afternoon, the busses going "home" to Sorrento can be so full when they approach Positano that they don't bother to stop at all!
Google is not good at ... everything. For example, they are terrible at scraping Business Hours from company and museum websites.