I just spent $450 getting the idle control valve and gaskets replaced as well as the front upper control arm replaced.
IAC - $65
Control Arm w/ ball joint - front upper - $42
or Bushings + ball joint - $27
Alignment - $60
So your $450 repair could have cost either $152 or for the luxury of a quick easy bolt in swap (entire control arm assembly) $167. IAC is a 5-10 min job, control arm assembly under an hour.
I bought the car for $2k and put $3k into the transmission.
Rebuilt transmission - $2256
or Rebuild kit + torque converter - $247
So again an easy $744 savings for a day of work or if you have an above average collection of tools and mechanical skill then $2753 savings for a two day job.
If you pay someone else to do every minor thing on an older car it becomes very expensive to pay that labor tax, and of course you get lower quality work, cut corners, potentially lower quality parts, and are inconvenienced being without your vehicle.
Personal anecdotes aside, at what point do car repairs become good money after bad? Never? Is it always cheaper-to-keep 'er? Anyone have a rule of thumb? (which takes us right back into personal anecdotes...)
Repairs are rarely throwing good money after bad. Step 1 before repairing a car is to buy one. Buy a car you would be happy with for the rest of your life, and preferably one that has depreciated about as much as it ever will and is in good overall condition so if you maintain it the value will hold in case life changes and you decide to sell it. Beyond that just do whatever it needs, and do as much of it as you can yourself. Occasionally some things requiring prohibitively expensive tools will need to be outsourced or you may find yourself in a time pinch when it needs something.
When buying a 10 or 20+ year old car you're going to have a year or two of catching up on all those years of neglect, after you've done all the proactive maintenance and repairs and fixed anything that has broken you'll have a very reliable car. Most vehicles are neglected and abused, people tend to not care to learn at least how to know different sounds/feels/etc. that are signs you need to check on something.