I have the same problem after buying a 996 and having my old Boxster built into a Spec Boxster racecar.
Around my area, car storage is $150-$200 per month.
My solution is definitely a lift. I am having one installed in the next two weeks. You can fit a Boxster (50") and a 996 coupe (52") in 9ft height. This includes the 4" thickness of the lift treads and another 2" to allow the lift to raise up to unlock the safety locks.
The lift I am buying costs $1,695. Delivery and full professional installation is ~$600 (of course, delivery charges will vary depending on your location).
The lift will pay back in 12-18 months. Even my wife couldn't disagree with this!
http://www.bestbuyautoequipment.com/4-post-car-parking-storage-lift-car-park-7-7000-lb-p/car-park-7.htm