So, I got about 2.5k visits to all my sites (I have 3 sites) this month and usage is about $0.8375. Also, there is somewhat a base cost as the nextjs app is running all the time so even if you get a lot of visitors, it wouldn't linearly go up, rather it would go up very less. However, there is a limit of 1gb storage in the base plan (should be plenty for maybe 50k page views, can't say for sure though as there is no way to check storage usage, however, it hasn't run out for me in the past 3-4 months of using it (7.5k page views))
Also, you can upgrade to the developer plan (you need to provide a valid credit card but won't be charged until you reach $10/month of usage per month and will only be charged for anything above that) which gives 100gb of storage for free.