You spent $785 over 21 batches. That's $37/batch.
But there is a flaw with your math - when I brewed the 22nd batch the equipment is now paid for ($37/batch, over 21 batches), so the value is zero. Hence, why non-term amortization is always over the equipments lifetime. Which, for an average brew rig that gets some nominal TLC, runs into the hundreds of batches - AKA, a few cents per batch.
If you calculate it your way, you repeatedly pay for the equipment. Amortization only works if you have a set term, or do it over the equipments lifespan.
You did not recover the cost because you have not calculated your cost correctly. Your cost is much closer to $1.20 per beer.
You've made a fatal mistake in your math - if calculating cost-recovery, you do not incorporate equipment costs into the per-batch cost - to do so is to pay for the equipment twice.
Again, my average costs:
Ingredient cost per batch: $25, or $0.42/bottle
Propane: $2.45/batch, or $0.04/bottle
Water: "free" (less than $0.01 in volume charge)
Total cost: $27.45, or $0.46/bottle
Store cost of 'equivalent' bottle: $2.25
Savings (i.e. applied to cost-recovery): $1.79/bottle
Much greater than that if you assign cost to labor of a 5 - 6 hour brew day.
Or much less if you subtract value-added. Of course, assigning hourly rates, or value-added, is somewhat silly for a hobby. The OP, and my blog post, were clearly about cost of operations.
In the states, 20L is equal to 676 ounces or 56 - 12 ounce bottles.
And since neither I, nor my blog, are American, one has to wonder why you'd take me to task for not using American bottles...
Once again, brewing is not about saving money. It's about enjoying the hobby and creativity involved in making your own beer.
You're assigning your motivations onto the actions of others. For some homebrewers, saving money could very well be the name of the game. Indeed, when I started 16 years ago I began brewing for the explicit purpose of getting my uni friends and I as drunk as possible for as cheap as possible. And we saved a lot. My log notes 23L (5 imp. gallon) batches of beer brewed for a whopping $6.38 - about $9 today. All of that using cooper kits, table sugar, and a free fermenter I found somewhere. It was nasty 'beer', but it served our purposes.
Thankfully, those days are behind me, but to assume there is no longer brewers out there like my younger self is . . . naive.
Bryan