The way I justify a repair on an old car is looking at how many months of payments on a newer car that would be. I don't buy extravagant vehicles, but I'd guess even a fairly modest new car or truck could run $350-400 per month. Plus the down payment. And then, figure what insurance would cost on top of that.
I'm sort of going through this same decision right now. I have a 1985 Chevy pickup that's getting rattier by the day, but it still runs fairly well. I had to put about $1,000 into it about a year ago, but not a dime since then. Well, I recently found a leftover 2012 Ram pickup, equipped the way I'd want it, for about $19,600+TTL at a local Dodge dealer.
I have to admit, I'm VERY tempted. But then, I look at the repair spreadsheet for my truck. At the end of September, it will have been 10 years since I got it from my Mom. And in that time, it's been averaging about $76 per month, which includes the $1200 I paid Mom for it, registration renewals, maintenance, etc. Everything but gasoline (9-10 mpg local, 15-16 on the highway if I'm lucky) and insurance (about $300 per year)
I'm about to put it in the shop and have a few things fixed on it. The parking brake broke, the driver's side power window came loose, and the radio has a short in it. Oh, and it has a huge crack in the windshield. Dunno how much all that's going to cost, but I figure if it comes in under $1,000, and I don't have to put any more money into it for the next 3 months, I'm even.
In contrast, just the sales tax on that 2012 Ram would cost almost $1200.
With this truck, it's a second vehicle, which means that replacing it with a brand-new truck would be, well, a bit extravagant! I'd probably make the decision to dump the truck if something really major failed on it. But, I don't think there's really anything all THAT expensive to replace on an '85 Chevy pickup. The transmission, if it failed, would be around $650-700. I guess if the engine blew, that might be the time to just dump it. Chevy smallblock V-8's are pretty cheap, but at that point I think I'd just bite the bullet and get a new (or newer used) truck.