Absolutely, I agree that it should not be that expensive, and I was not saying that you should buy fancy commercial mixes, I was just pointing out that there are some that are pretty decent, and that some of the almost dirt cheap ones are actually quite poor in quality. So in that sense you still get for what you pay for.
So, while at least twice as expensive as I would like to pay, the difference between the cost both in time and investment in components, then the need to get those components in the right size (your cutting up coconut material, but sieving out the fines or too coarse parts comes to mind as well) all adds up to something that when all is said and done if you'd put money totals on it, you'd not be as far off the commercial mixes in price than you would think. The willingness or ability to invest your own free time into a project like this maybe something you'd do for free, but for a true cost benefit analysis you should count the time spent as a cost (since at the least that time investment is less time spend doing other more fun (and useful?) things with your plants) and put reasonable money amounts on your components, which will not always be free. I think we'd all be really happy if every plant we have would grow great in a pot to which we just added dirt from the yard, but I can tell you that for most plants that is not the case at least not where I live.
In my case with limited time to spend on my plants period (because of work, the rest of my life, and especially this time of year the 105+ Ts making working outside less than pleasant) makes me want to spend as little time as possible putting a soil mix together. Right now I mix a quarter bag of pumice with a half bag of cactus and succulent soil mix and I am done. I will probably have to go back to the $6-7 / bag mixes because of how many plants need to go from 1 to 2 and 3 gallon sized pots, but I certainly wish I did not have to.
