Bandwidth limitations are individual per plan and generally larger for the larger plans. The current usage can be viewed in the server list: https://dash.99stack.com/resources/servers, on plans where bandwidth limitations are in place.
Bandwidth overage has a fixed price of $0.05/GB/month. This only applies on first generation plans, later generations are not subject for bandwidth overage fees. Additionally, a monthly limit can be set in your settings page. The limit will kick in and block network access to your servers if you don't want to pay any extra for additional bandwidth. Bandwidth overage will by default be deducted from your balance, one GB at the time and instantly.
Plans without limits
Currently, the only plans subject for bandwidth overage fees are the first generation plans. Later generations has a slightly stricter fair use policy instead which means your bandwidth usage is unlimited but should be balanced, this is recommended for all types of server applications except maybe downloading servers, CDN's and similar where bandwidth usage tends to burst.
How bandwidth usage is calculated
Bandwidth usage is the sum of incoming bytes and outgoing bytes. Each plan has it's own individual limit, all usage up to that limit is included in the server price and will not incur any additional fees. Second and third generation plans does not incur any additional fees on bandwidth overage.