$3.50(cost per million calls)*15 = $52.50
Lambda is pretty cheap.
For example a 256 memory function running for 300ms being called 15,000,000 times would cost 21.77.
All together that's $74 a day for just lambda and API gateway without any extras (cache, bandwidth pricing etc).
Maybe more expensive that raw infrastructure but it's a pretty inconsequential amount of money per day for close to no ops.
Practically any smaller instance type (i.e. m3.medium) can handle this small of a load all by itself, without even breaking a sweat.. and instead of paying $74 per day, it would cost less than $74 per month.
In fact, ELB + an ASG of three t2.micro's running continuously would cost around $49 per month, not per day, and possibly around the same amount of effort (or less) to create/maintain/manage.
It's somewhat apples and oranges, but there's no doubt that lambda is expensive compared to plain old EC2, and that cost disparity increases linearly with scale.
But you have api management to sort out and versions to solve which api gateway can do fairly easily.
API gateway is connected to Cloudfront for low latency.
You can simply add a cache for your API.
You have analytics already setup up and ready to go.
Also other things like API keys, auth and cognito integration with other integrations etc that API gateway has.
You can deploy and maintain tens of lambda functions fairly easy, to get something similar you would either have to use some container service like ECS or Kubernetes so have to figure them out compared to just deploying your code with one of the frameworks out there for lambda.
I'm not looking to put down Lambda, although it could maybe be a bit cheaper; we use EC2/ELB/ASG extensively with Userify but we might use Lambda for eventing-based services in the future. Evaluating each on its own merits will probably give you the best picture of what's right for your project and team.