We get this question all the time. In fact, a good friend of mine just asked me this question a few weeks ago so I'll tell you what I told him...if you consider only the dollars and sense then yes.
I've calculated this a few times over the years and I always get the same answer so I'll share it with you here. It costs about $400 per month for depreciation, interest, maintenance for the first 4 years of an average vehicle's life. It costs about $250 for depreciation, interest, and maintenance for the next 4 years of an average vehicle's life. So, it's about $1800 per year cheaper to maintain and repair a used car than it is to buy a new car. This difference is even more pronounced for trucks because the depreciation is so much higher in the first few years. It is almost always cheaper to fix a truck than replace it.
So, that's how I do it. I buy vehicles with 100-150k miles on them and I fix and maintain them whenever they need attention. When my needs change I buy a new vehicle. Otherwise I just keep fixing them. It's hands down the least expensive option.
The best way to do this is as follows: Find a trusty repair shop. Have your vehicle inspected as soon as you buy it. Update any fluids if you don't know when they've been changed. Fix any belts/pulleys, leaks and suspension parts ASAP and then run it. Do all your oil changes at that shop and fix any items as they appear on an inspection so that a small issue doesn't turn into larger one. Rinse and repeat.
While this advice may seem tilted towards the shop I can assure you that it is by far the cheapest way to buy and maintain a reliable vehicle. We've seen $250 wheel bearings turn into $1300 jobs because customers don't follow this. We've seen $150 differential fluid changes turn into $2500 differential replacements. Keep the monster at bay and feed him small bits frequently. :)