How do you spend $1K+/month on a car? I've known a few people who pay that much, but they were trying to insure a sports car below the age of 25.
Buy a simple used car in good shape, with 30K-60K miles on it. This will cost $7-12,000, and last for at least 8 years with a bit of maintenance. Figure $60-150/month for gas, $100/month for car insurance, and you're still in the area of $350/month. Add a few hundred per year for tires and repairs. When the repair bill reaches $100/month, buy another used car.
Buying a new car, a fancy car, or a car with poor gas mileage will make these numbers much worse, of course.
Rather trivially, even without insurance surcharges. I had a Lancer Evolution VIII MR for a few years...
* Insurance was $150/mo. This was without an age surcharge, no speeding tickets, no accidents. As low as it was getting.
* Payments were ~$500/mo for 60 months at 4.9%. That was after putting about 23.5% down, and taking sales tax into account.
* Gasoline? This was a car that got 4mpg when you caned it, and averaged about 14mpg. I drove 500 miles a week. When premium was $3.50-$4.00 a gallon. Gas was $500 - $700 a month, as 150HP/liter doesn't come for free.
* Maintenance? I changed my own oil (synthetic) every month. Even DIY, that was $40/mo. Nevermind everything else. A set of brake pads (DIY) was still $500-600. Rotors? More like $900. I ended up doing two brake jobs in 3 years.
* The summer tires, between track days and usual commuting, lasted ~12K miles a set, at $1300/set. I also had dedicated winters at $900/set, but those never needed to be replaced. But basically, a new set of summer tires once a year meant $110/mo in just tires. "Cheaper tires" weren't an option, unless I wanted to completely neuter its handling characteristics.
Honestly, it was close to $1500/mo to run that thing, and that was simply a $36,000 car that could happen to hang with things 2 to 3 times its price on a road course. I was contracting for years, and never got anything close to where I was. If I was commuting to the same place, I would've absolutely moved, and saved $300-600/mo in fuel/maintenance quite easily.
I have a Miata (MX-5) now which costs dirt to run, mostly because mileage is all fun, and < 4000 a year. Insurance is cheaper than an economy car. Gets 24mpg driving it like an idiot. Proper tires are only $750-800/set. Maybe costs me $150-200/mo, since it's paid for.
Just a point: For that car and what you apparently used it for, a car co-op wouldn't work in any case. I doubt zipcar lets you take their cars to track days, even if they had a car you'd want to take. And at 26k miles/yr, I don't think a co-op would work either.
Rather than "trivially" making it to $1500/month, I think you pushed most parameters to the expensive side.
As a contrast, I have a 2001 Passat that I've owned for 6 years and do all the maintenance on myself. Over 6 years I'd estimate it's cost me $25k total, which ends up being $350/month. But then I've only driven 40k over those 6 years, too.
I wasn't looking at it from a co-op or car sharing perspective. I was looking at it from a "how can a car cost $1000 a month perspective". The Mitsubishi Lancer Evolution and Subaru WRX STis are probably the best known examples of post-purchase sticker shock considering a transaction cost that is only 110-115% of that of the "average new car transaction cost". Especially the Mitsubishi, which was referred to by Jeremy Clarkson as "needs to be serviced every 300 yards".
The Yokohama Advan A046s had a 140 treadwear rating. High-output turbocharged engines (140HP/liter bone stock) tend to require frequent oil changes. High-output turbocharged engines also tend to run rich, basically using unspent fuel to help cool the turbo -- 14mpg is seriously what was observed under mostly rush-hour highway traffic. Premium fuel was required since it still had a near 12:1 compression ratio.
Those cars would never be offered via co-op, and I can't think of a car sharing service that even offers genuine bona-fide sports cars. They are rare, not mass market. They're incredibly noisy. Not very comfortable. They're "compacts" (in the US). They're only offered with manual transmissions (until recently). They'd be absolutely catastrophic loss leaders, especially with hooligans beating the piss out of them. They were engineered to be the most entertaining cars for the lowest transaction cost possible... but they very quickly compensate for their low sticker price with maintenance bills from a 911.
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What about those who purchase, say, a new Toyota V6 Camry, load it up to $30K, then put the minimum they'll allow down? Right there you're looking at a $550-$600/mo payment (and dealers won't care if they're getting paid and have the car off their floor plan). Add $100-150/mo in insurance. Assuming a 25mpg average, it's not hard to get right on the cusp of $1000/mo for the first 5 years, depending on how far people commute. That's with a vehicle that's around the average transaction price for a new car in this country.
My point remains that it's not hard to hit $1000/mo. I don't consider that prudent in the slightest, but a fair number of people experience just that.
As I said, I know how to not spend money on a car as well. I bought a Mazda MX-5 cash. Moved close enough to work to take mass transit. Negotiated secure garaging in my lease. Just use it as a toy. Between insurance ($120/mo), maintenance and fuel, $200/mo would be exceptional. I think I've spent $600 total on gasoline in the past 22 months, whereas 4 years ago, I remembered spending that every month on just fuel.
I pulled that number from my cubemate, who's picking up a used Camry (relatively recent, '07-'08)... because apparently you can't get married unless you own a car.
$1K is payment + maint + insurance + gas. Hell, I've gone over my parents' finances and that's roughly in line with how much they shell out for their cars (note: Mazdas and Toyotas, nothing fancy). Around where I grew up, unless you're driving a junker, $1K a month is pretty easy to hit (last I checked, my parents with their perfect driving record are doing ~$250 on insurance alone).
Buy a simple used car in good shape, with 30K-60K miles on it. This will cost $7-12,000, and last for at least 8 years with a bit of maintenance. Figure $60-150/month for gas, $100/month for car insurance, and you're still in the area of $350/month. Add a few hundred per year for tires and repairs. When the repair bill reaches $100/month, buy another used car.
Buying a new car, a fancy car, or a car with poor gas mileage will make these numbers much worse, of course.