I am not even thousands km near the level of what you are doing, but my client was paying $100/m for an AWS server, SQS and S3 bucket, for a small PHP based web application that uses Amazon Seller API, Keepa API for the products he ships. Used MySQL for data storage.
I implemented the whole thing in Python, Django, and PostgreSQL (initially used SQLite) put it in a $25/m unmanaged VPS.
I have not got any complaints about performance, and it's running continuously updating product prices, details, processing PDF invoices using OCR, finding missing products in shipments, while also serving the website, and a 4 core server with 6GB RAM is handling it just fine.
The load is not going to be so high to require AWS and friends, for now. It's a small internal app, probably won't even get over 100 users, and if it ever does, it's extremely simple to migrate, because the app is so compact, even though not exactly monolithic.
And still, it probably won't need a $100 AWS server, unless we are scaling up much larger.