I recently was reading this comic from xkcd, and it got me thinking. How does one determine how much one's spare time is worth? It seems to me that in particular, a person on salary has a fixed income, and thus it's not like they could work for an extra hour to make up the money, thus it might almost be worth it.
If you enjoy driving 5 minutes out of the way to get gas (or mowing the lawn or whatever), then it is perfectly rational to do it. If not, the value of your time is how much you would value (not necessarily how much you would get paid) doing something else.
MrChrister is right, the concept behind the comic is opportunity cost. In a nutshell, if driving an extra 5 minutes for gas is complete drudgery to you and you would only do it to save money, may as well get a part-time job at minimum wage instead. If you would rather, say, go watch TV instead of getting that part-time job, then you value TV-watching at greater than the minimum wage, and it is still irrational to drive 5 minutes for the gas.
Basically, the answer to your question is to figure out what else you could be doing that offers similar overall pleasure/pain to the task at hand and see how much you would get paid to do that instead. It doesn't matter that you are on a fixed salary.
This really is different for every individual.
Years ago, I saw a guy in line at Costco to return a pencil sharpener. The thing cost about $20. Forget the gas. I recognized the guy, he was a mutual fund manager with a million dollar plus salary. But I guess he valued his own time at far less on Sunday than the $500/hr he made during the week.
As your question suggests, the starting point is your wage, but for extra money, it goes down from there.
The question has merit, I just think that few people stop and do the math. There are also some things people just enjoy doing, like mowing a lawn, which I'd rather pay for. Often, people will pick and choose that way and not really decide whether they are working for themselves at a certain wage.
I think the key to this question is to think about how people tick. The underlying assumption in any decision that measures monetary value or other economic principles is that we're rational actors in a global marketplace.
Life isn't a ledger or a taxi on a meter. If you have time on your hands or enjoy the act of driving when you run your errands, that diversion to save a dime on gas isn't a net loss, it just is. That may be a different story if you need to get your rental car returned so that you can fly home -- in that case, you're ok with getting hosed by the gas station strategically located between hotels and the airport.
Our motivations are deeper than that. Some people get a feeling of satisfaction or security from always getting the "best deal". Some folks visit a particular gas station because the clerks are friendly or the coffee is good. Others want to pay more -- they buy "premium" gas from Exxon/Mobil because they are premium people who "buy the best".
I'm with several of the others answering the question in that it isn't about opportunity cost so much as the value of the time savings.
For example:
I'm sure I could re-shingle my roof and save a ton of money on labor, but the marginal cost of having someone do that for me in the 100+ degree Texas heat makes it worthwhile to outsource that job.
How much I could have earned at my job for however many hours it takes doesn't really play into it unless I am specifically giving up an opportunity to earn money instead of working on my roof. For example, it might tilt the balance more towards outsourcing the roof job if I had an offer for contract work at $100/hr that I wouldn't have time to do if I worked on the roof. Also, considering the time I'd probably take off from work to do the job is paid vacation it isn't like I'm giving up some of my paycheck to do that job. The real variable is how much value I assign to being able to NOT work on my vacation days.
The bottom line is this:
What your time is worth is variable and depends on the REALISTIC opportunity cost of using that time on a money making endeavor instead of the option you are considering.
I guess you could figure it out based on your total income, and the total number of hours it takes to generate that income if you want to do it simply.
Count you job, side work, soda can deposits, and saving earned directly by effort (coupons and deal shopping)
But the real answer to the question is understanding Opportunity Cost and what you could be doing instead. The problem with opportunity cost is the value system that judges the worth of the other opportunities is a deeply intrinsic factor that cannot be judged by anybody else.
For me, I factor in a couple of things.
First of all, there are occasional opportunities for me to get more money at work, that happen infrequently. If I'm in one of those periods, I pretty much value my time at my salary, as far as doing something that I'm not great at or enjoy. I also am more likely to buy food to fill me up, park in a parking garage instead of walking 15 minutes to park for free, etc.
But, when I don't have extra incentives, I tend to think more of my budget, and think about things from there. I might spend 4 hours to fix my microwave, when it might come down to only saving $50-$100, even though my time from work would seem more valuable.
I think it depends on how much time I am saving and what I am saving doing. I do not go out of my way to save a few pennies on gas. But If i know that I will be traveling somewhere with cheaper gas anyway I will wait to get there to fill up. But I if I need gas before there I will fill up at the place i stop rather than just getting enough to go to the new location.
I heard it best said as being dollar smart instead of dime foolish. Going out of your way for gas like this is dime foolish. On the other hand if you can bundle your trip to the store nearby or other tasks it certianly adds to the value.
