Seems to me that it depends mostly on how much of the GPS data is requested by apps (more than you'd expect actually). As long as the GPS switch is on, GPS is listening to keep track of your position. The radio calculates your position (in degrees of course). When no apps are polling for your GPS position (it's in a passive state), the GPS radio won't push it's position to Android (since it's not needed) and power consumption is about the same as your communications radio (phone calls/texts). Actually, the GPS radio switches to a less accurate mode, keeping track of your approximate position. The initial position calculation takes a lot of calculations (since you've got no reference point at that moment). Position shifts afterwards require only a fraction of those calculations.
When an app is polling for your GPS position, it sends a request to the GPS API, and the API in his turn tells the GPS to listen for an exact position. Of course, this uses a lot more power. At this point, GPS keeps tracking for its exact position for a while until there are no more requests for a certain amount of time or (obviously) the GPS switch is turned off. This way the GPS radio tries to avoid initial positioning calculations on subsequent position requests (the same story about reference points). When you set the GPS polling timeout for an app too high, it requires initial positioning for every poll, draining your battery a lot faster.
Turning your GPS switch on and off a few times in a short period will drain your battery very fast, again because of the reference point stuff...
Actually, the GPS radio is only completely off when your phone is off. As long as your phone is switched on, the GPS radio is at least in a deep sleep mode, listening for instructions to get activated (when the GPS switch is turned on). In this state, GPS strictly still consumes power (however the rate of power consumtion in that state is too low to be taken serious by at least 95% of the people out there).
Conclusion: having the GPS switch turned on isn't a huge battery drain as is. The extra drain is mostly caused by apps requesting GPS position. I don't only mean your nav app, but all location based services, like local weather widgets, google location-based search, location-based automators that use your GPS, Facebook, ... The list of apps that use your GPS at some point is really long.
Leaving the GPS switch on is not a bad thing or a huge battery drain, but if you do so, you really should review the location settings of your apps. I've turned auto-updating off for almost all of my apps.
Cheers!
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