If your looking for someplace where you can easily access your garaged car in Manhattan for less then the commercial monthly lots/garages, your not going to find it. Maybe someplace in Queens or deep Brooklyn as far as getting to it by public transportation. If you have a daily driver then your options open up depending how far you want to drive to get to your Porsche. There are some "boutique" heated etc. car storage spots about a hour out of the city in CT. and Westchester County, some may be near Metro-North train if you don't have another car. I've read that these tend to be around $250 a month. A couple I've found in PCA CVR's monthly magazine are:
thethirdgarage.com in Norwalk CT.
SoNoGarage.com in South Norwalk (2 blocks from train)
Home | Collectors’ Car Garage® | A country club for car in Bedford Hills NY $450 a month
Services | Auto Babysitters -Brooklyn
https://www.tistorage.com/car-storage -Brooklyn
Unfortunately unless you have a friend with a garage or warehouse just outside of the city your going to pay a premium. I would join the MetroNY PCA chapter and see what members are saying. Believe it or not Craigslist might be your best bet.