A lot of hype today surrounds the word "firewall"; some of this is corporate-fabricated and some of it relates to the actual functionality of a firewall. The commonly purveyed misconception is that a firewall is some sort of magical network security device that will put an end to every administrator's security concerns. This couldn't be further from the truth; a firewall is designed to provide advanced IP services such as packet filtering, port forwarding, and network address translation. This said, a correctly configured firewall should be a part of every secure network. It's just not the be-all and end-all of network security.
For estimating the price of custom firewalls design we determine the type and amount of professional time to meet the requirements. The estimation of the type and amount of professional hours for a task is made by someone who is familiar with the service.
For a quote we may require a retainer fee which can be applied to the cost if we are asked to proceed. Because requirements may not be clearly defined or may change for custom work we may quote with milestones to allow mutual renegotiate between milestones if requirements change.