guides for trekking in PNG

How essential is it to have guides for trekking throughout PNG? Isn't the Kokoda well traveled enough to go without. Maybe just you and two or three people?
The more I look into a trip it seems guides are going to be one of the most expensive things. Will I really need one everytime I do something while not on a tour?

I can't comment on the Kokoda from personal experience, but it is certainly well-travelled.

As for the rest of PNG, I really think a local guide or porter is generally necessary for longer treks (both for route-finding and security), but away from the few hikes popular with tourists they shouldn't generally cost too much. Just hire them at the actual starting point of your trek, not via some tour-operator in advance, if cost is your priority.