How do you manage you MS Project Licenses?

Several of our Project Managers need Microsoft Project. Computers get replaced and staff changes are continual. Question 1: how do you move Microsoft project from one computer to another without running into licensing issues. Question 2: Boss doesn’t want to spend $599 a crack for each new copy of Project. It doesn’t appear that MS Project can be purchased in 5-pack or 10-pack. How is the MS PROJECT PRO different? Does it allow multiple users to create/modify project files interactively? Is it more difficult to use? Would it be the solution we need to run projects for multiple managers more economically (cheap)?