If your needs are large enough then that means you should probably get your own AS (like an ISP) and sign up with a tier 1 provider for the peering arrangements, and you buy/rent servers preferably in multiple locations. There are more options than AWS. Regardless of you use you can negotiate discounts based on volume.
To give you an idea, I run my own VPN service using a VPS that costs $5/month. My monthly traffic is more or less your estimate of 300 Gb/month. My provider does not charge for extra unless consumption really goes through the roof.
This an end-user rate for just one user (me) and it's already extremely cheap. In theory I could add a few subscribers on the same machine and rent my VPN service, and quality of service should remain good. If I scale up this arrangement, the cost per user/month becomes negligible.
Just shop around, the market is very competitive.
Providers such as AWS, Cloudflare offer extra services like DDOS protection so you also pay for that assurance. It's built in the price.
Hosting is cheap, but monitoring and upholding quality of service increases costs. I don't mind occasional downtime since I am my own customer. If I had paying customers obviously I would need to invest in resiliency.
That said, many hosting companies are resellers, may do not own any infrastructure at all but rent everything to provide services to end users.
VPN providers are no different.