Sure a self-hosted pi is cheaper, but that's an apples vs oranges comparison.
This is what you pay for in the datacenter. The request handling itself is much simpler.
I'd even question whether or not the application needs to be in an "AWS-grade datacenter" since it does 1 query every 10 seconds but sure...
I'd be more than willing to bet hertzner's cheapest VPS (~$3.25?) would do over a million[1] requests per month pretty easily and they're inside a data center which is a pretty apples to apples comparison.
My point is that the cost per request is high, regardless of data center or not. That's a lot of money for such a low volume of requests.
[1] Which is still only a request like every 2 seconds.
Not that it has a gigabit uplink. But handling 100k req/s is trivial for a well-written server.
Hertzner is closer, but even then, it's not AWS Lambda. Setup and maintenance of a VPS is not "free".