I'm getting a re-roof and have both a ridge vent and a fan that turns on automatically. Do I need both or can I save some money by foregoing the ridge vent?
It takes power to run a fan, and power costs $$.
Passive (ridge vent, wind turbines) costs nothing after the install.
A properly installed ridge vent---on the style of roof that is recommended(gable end roof) with sufficient open soffit vents(An absolute necessity) will ventilate an attic space naturally for no cost other than the cost of installation.
Ridge vents vent the hot air in summer or winter at the highest point of the roof---the peak by utilizing the fact that hot air rises---and rising air creates a low pressure area under it----where the soffit vents are---pulling in cooler air to replace the escaping hot air. The only way to stop this from happening is to physically block either the ridge or soffit vents.
The only down side to having a ridge vent installed is it does change(slightly) the profile of the ridge of the roof. A properly built ridge vent will not allow rain/snow/insects into the attic.