If you would have the same setup, but with EC2 instead of lambda, you would still have the same CloudFront pricing, so you should compare the $410 lambda costs for 100.000 customers with the costs of some EC2 servers plus ELB. Of course, then, you will miss the benefits of 1. no server-maintenance, 2. no down-time or scaling issues and 3.  initial startup costs - without customers.