Sounds like they behaved exactly how you expected them to. You can't get upset because the threshold was passed.
$2,500/mo to host a static site is ludicrous.
But it's not to host a static site, it's one click CD, multiple environments, global CDN, and a few more things. I'm not going to say how mu h it is worth, but if you just want a static site with a domain you can do it for free/sub $10 on many many providers.
What can you do for $2,500 on Netlify you can't do on Vercel for $20/user/month?
I suspect that if you're using these "as expected", your bill on both sites would be the same.
[0] https://www.netlify.com/pricing/ [1] https://vercel.com/pricing