Serverless Solutions on AWS

Our customer, an education sector major player, have their workload on AWS. About 2 years ago, we developed Asp.Net based Web API that customer deployed on their API Platform, as a set of Rest APIs to be consumed by a mobile  application, with possible addition of API Consumers in future. Their Annual AWS bill for this application was around $ 15,000, although not a big amount for customer, but, customer was worried once they start adding  more API consuming Partners, their AWS bill will increase.