Can a complement good be free or have a fixed cost? I was not around at this time but I know roads(existing for bikes) and gas(existing for tractors) were complements for cars.

I was looking at complements for computers and noticed the ARPANET could maybe be regarded as a complement because it appears it would increase a computers usefulness.  I'm trying to leave the history out of this but I figured an example would be helpful.

Can a complement good like the ARPANET have either a fixed cost or be free?
Air is a complement good for a lot of things. If you get no air at all then food, gold and iPhones give you no utility either. And air is free! Except in Spaceballs...