It's not always a good idea to add up all your receipts and see how much money you have sunk in your current project. Still knowledge is power or at least awareness. I've got a little over 5,000 dollars in my Mustang, probably more that I can sell it for, but every car will cost you money. If you don't spend some money on repair and maintenance you won't have any kind of reliable car.
My 96 Mustang has got more into it that my 70, but it has rewarded me with lots of good fun and reliable service. When I bought that car it was in pretty good overall condition. The paint and interior were good and even the A/C worked for the first year. The biggest problem was a worn rag top with some duct tape patches. and the rear glass pulling away from the fabric. I reapplied a better looking black duct tape repair and drove it for another year while I saved up money to replace it. The top itself from a repop supplier was only a few hundred bucks but the labor cost is the issue. I had installed a new rear window and top in a previous car but the results weren't that smooth. I found a local shop that would replace the top, labor included for a grand. Did it in one day too. I have been quite satisfied with the results. That was probably around three years ago which has helped to amortize the cost. This car was in much better shape that my 70 which pretty much needed everything replaced. Didn't have to mess the paint or body work, or interior, no rust or botched up repairs to redo. I later had to fix the A/C, reseal the transmission, replace the battery hoses, intake manifold, fuel tank grommet, brakes and rotors,tires, new CD player and just a couple of months back, a new fuel pump. That was 500 bucks alone. But I really like this car, it fulfills all my performance parameters even though it's twenty years old. It's plenty quick, handles great, sounds great (Flowmasters)! Cruises effortlessly at 70 mph at 2,200 rpm and will return 25 mpg. at 70 mph. on regular gas. And the top goes down. Just a great car and a great bargain. But...
On the other hand my 70 is a cool looking car and a real Classic but in it's current configuration the driving experience is pretty blah. It was built to "secretary special" specs. Six cylinder motor, ( though it is the 250ci.) auto and power steering and that's it. Manual four wheel drums, of course no A/C cruise control, or tilt wheel. It rides okay and will handle pretty good now but fuel economy is dismal. I got 15.5 mpg at 65 mph. cruising down to Santa Maria for a car show last May. Still it was reliable and made a 560 mile trip without incident, which really is worth something. It's really easy to work on and parts are readily available and cheap. And people really love to see the going down the road! Thumbs up, big smiles and lots of questions at gas stops. The fun is basking in the Classic Mustang Mystique. So many people have owned one, or their Mom or Dad, Uncle, Aunt, sibling, friend, neighbor, co worker, etc,etc. owned one, or wanted to. I can build this car up any way I want to depending on my tastes and finances. On Sunday I was driving on the freeway with my son, who is also a car guy. I told him, " You know, I could drive this all the way to L.A. ( then a pause), but why would I want to?"
Time will tell.