If you have a small number of requests over a large period, serverless is cheaper (that includes an api that is idle most of the time and peaks at a very high rate occasionally).
But once you go live and start to grow, there is a point where the lambda cost equals the instance price. And with an instance, you can almost always do a lot more than that number. Again, that will work if your requests are predictable enough that you can see this number consistently over a large period, say, every week at least.
