It really depends on the type of budget you have and the size of the network that you want to monitor. I have run a Linux/Unix page counting system (LPR) as well as running Printer Accounting Server by Software Metrics that runs on an NT machine.
If you have the patience and knowledge to set up a Linux page counting system, including programming the counting/monitoring system, it is definitely cheap. I prefer the Printer Accounting Server by Software Metrics, however.
NOTE: There are other packages by other companies that may be cheaper that PAS, but you'd have to look around. It is the only one I've run on an NT network.--Also, PAS will not run on Active Directory yet (estimated release in November) .