I did not test this on my own. But if I would have to implement it, I would use a very coarse resolution (4km for example) and poll that about every 10 minutes. You can easily do that with cellular triangulation (an information that the iPhone has anyway even without doing anything aside from being a phone).
If there is a geo-fence within that region, I would increase the polling rate and also increase the resolution. At some point you will have to switch on GPS to see if you're within that region for the reminder.
So from a power point of view the power consumption depends on the geo-fence size that you set and the distance to that fence. If you're closer, the power consumption will increase. If you're away from any interesting point the additional power consumption would go down to 0.