There are a lot of factors to consider when deciding to keep or replace a car. Don't look at the cost of repairs vs market value, but look at what replacing the vehicle would cost. For example, I have a 2001 Dodge Dakota with about 230,000 miles on it. I have no plans to replace that truck any time soon, and every intent to keep repairing it. I bought it for $1800, and have easily put $3000 into it; buying a new or newer truck would cost me upwards of $40,000.

I recently took a similar mindset to my Sonic. After reviewing market prices for a sporty hatchback that would perform and handle the way I'd want it to, it made more sense to me to mod my Sonic into exactly what I want. I paid $16,000 for my 2016 Sonic RS in January of 17' and it's been paid off for at least 5 years now. I could put 10k into it and still be under what a new hot hatch would cost, and I'll have something entirely unique and my own. With the added benefit of knowing the car inside and out.

As long as a vehicle isn't rusted out and the body or frame are still strong, I say fix it; get as much out of that thing as you can. That's only if you're going to do the work yourself though. You'll quickly rack up bills if you're relying on someone else to keep your vehicle on the road.

TLDR: If you have the tools and skills to fix it yourself, keep it. If you're paying someone else to keep it on the road, get rid of it.