So, $2,257/mo for 15Mreq/day ≈ 174req/sec

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.