Interesting question: who deems what a job is worth to society?

Many things:
First, we all do. As a simplistic example, would you hire me at $50/hour to patrol the front of your house for squirrels? No. No matter how diligent or hard-working I was, that function is not worth $50/hr to you. In fact, it may be worth nothing at all. A little less simplistic: would you keep me on fulltime staff to clean your apartment or house at $100,000 per year? Probably not. If I charged $1,000 per year, you probably would. A clean apartment/home is a good thing (by most people's standards). It has value. How much value? Well, that depends on how much people are willing to part with for it. Maybe you would rather waste your time cleaning. Maybe I would be idle most of the time because your place isn't that big and therefore it doesn't justify fulltime staffing.

Second, alternatives. I touched on this already a little with your substituting your labor for mine, but it goes even further. Can the process be mechanized? In the era before computers, there were still things like directories and it must have been a damn tedious process alphabetizing all those names. The person was, no doubt, hardworking, but as computers came along and could do the job quicker and more accurately, well, that person no longer had function. Yes, the value of sorted data didn't change, but the cost to acquire sorted data did change while that person didn't (or maybe they did and became a valuable computer technician or something).

Third, output. What is its intrinsic value. A car has intrinsic value, but it also varies by person - a car is worth more to the President than it is to me. He's busier, I can take the subway without fear of assassination, etc. The more expensive a car is, the fewer people will have it. For the most part, producers don't get to price discriminate - if Bill Gates goes to the Mazda dealership he'll get a similar price that I get even if the car is worth a lot more to him.

Forth, competition. Are there other people that are willing to do the job for less? I'm guessing beer tasters don't get paid well, but I'd also wager that breweries don't have trouble finding people for the job. Likewise, if competitors are more efficient (either companies or other workers), it makes your labor less valuable. If your company produces 1 car for every 50 hours of labor and another company produces 1 car for every 25 hours of labor, your labor isn't worth as much. If I can automate processes that save time, my labor becomes worth more and yours less.

Fifth, other goods. Economics is about allocating scarcity. Many try and argue that we're not an economy of scarcity anymore and that we have enough for everyone and that it's just a corrupt capitalist system that prevents that from happening. Then they create cost of living calculations for a living wage that consider someone without cable to be in dire poverty. Face it, we like things. If we could all live without things (from TVs to medicines), we'd be fine. So, the question is: on a per dollar basis, how has the automobile fared against other things we can spend our money on? For me, on a per dollar spent basis, my computer is much more valuable. Are people moving a certain percent that they used to spend on cars toward CDs or TVs or computers or cranial piercings? I have no idea, but we all allocate our budgets in ways that we hope will increase our happiness. People's tastes change, society's tastes change. Portable radios do positively affect your happiness and are cheap (say, $20), but people seem to have widely pronounced that MP3 players (at 5-10x the cost) positively affect their happiness at a rate greater than that 5-10x increase. Or they're just idiots that don't know what to do with money.

I could go on, but I think you get the point. It's a truly fascinating topic. In the GM case, there are some neat applications to see.

1. When GM, Ford, and Chrysler were pretty much the only auto makers, the amount of competition was less which meant that consumers would pay more and fewer units would be shipped. If you want to calculate how this works, look up the cournot/nash equilibrium. As the car companies were selling at higher margins and ignoring the poorer people, labor could demand more money since the car companies were making more money per unit of labor used.

2. Other firms with lower labor costs also became more efficient. Toyota makes cars faster combined with a lower cost per hour. That means GM's labor is worth less money.

3. The value of output might be fluctuating. Here is where I'm spouting complete BS. As gas prices rise, the intrinsic value of a vehicle declines. Americans also seem to be having a shift in their perception of public transit and as their view of it becomes more favorable, the value of a car decreases. However, as most people are suburban, a car can still be a lifeline.

It's late. I'm done. Hopefully that was interesting. It wasn't meant to have any view pro or con in it, just more of a view into how prices get set and how we value things.