You miss the point. This discussion has nothing to do with hobby time. If your hobby is making widgets to mount on a tractor that's great. Don't then turn around and say, "hey guys I just built a corn fritter for my tractor for nuthin cuz I made it out of my Brother in Law's Yugo". That is no longer hobby time but tool making time. You may have had a ball doing it, but you just "spent" time in the process of adding to your tool chest. Now you own something that you did not have before. It definitely cost you. It is absolutely no different than working the hours for a wage that it would cost to buy it.
So now you are basicly "Lostcause, Inc." and no longer a hobbyist, so you have to consider your overhead, materials, labor, property tax, blah blah blah in your hourly rate.
actually, i think my point may have been missed. what i'm trying to say is that there is a 95% chance that the person who spent 100's of hours converting said yugo into a farm implement did not have a limitless fund of cash to do it with. they didn't leave the 5 mil in the bank and spend all their time to create a $5000 implement. likely, they were in a position where they could not justify or afford the $5000, but they were able to come up with free time to replace the cash. it's not free, you either spend a lot of cash or a lot of time.
let's say you wanted something that cost $5000 if you were to buy it. the alternative is spending $1000 in materials and maybe a horrendous 400 hours in labor. you're going to look at that by saying that the person is crazy because they only paid themselves $10 an hour for their time. another person will look at that and say that they are able to do this and save $4000. they are likely to do it because there was no easy and fast way to get the extra $4000 by working more, or taking a second job, or maybe they just don't like given profession enough to want to spend any more time at it than they already do.
everyone's time (and money) is valuable in different ways is the point i am trying to make. what you consider to be a wise use of your time is not the same for everyone. the last 20 years i spent working at a career that was sometimes too much for me. i worked night shift for the extra money, i worked weekends for the overtime. i worked out of state and out of country for months at a time to try to get ahead... i've just turned 40 and i can say that regardless of the time i may spend on a project, my other half would certainly rather have me at home and out in the shop as opposed to off working the way i used to.
i am not blind to the cost, so i do weigh the time spent against the cost. if it's a cookie cutter item that can be purchased for a few hundred dollars, then i'd likely not do it myself. however, if it is something i want that will cost several thousand, i'm likely to make it myself, because i can keep the same lifestyle i have now, and not have to outlay all that cash.
as for it not being a hobby, i think that's a decision for each person too. if you're an avid golfer, you'll spend a decent amount of money on new clubs every few years. greens fees every weekend, wear and tear on that big suv that clutters up the parking lot, and if you're a really elite golfer, course memberships running several hundred to several thousand dollars a year. my "hobby" shop consists of an old lathe, old mill/drill, small band saw, couple new(ish) welders, torches, and big toolbox of other odds and ends collected over the last 20 years. the big ticket items (lathe, mill, saw, welders, torches) have totalled about $3500 over the years, and the cost of consumables is probably a couple hundred a year. i could easily spend more money golfing. i certainly do consider it to be a hobby, because after 20 years of a desk job, i enjoy being able to spend some night and weekend time working with my hands.