Paul Mags the author here.
Thanks for the kind words.
A few points
1) I have seen cheaper externals, but online prices are consistently ~$40. I'd rather be consistent than say "At a thrift store in XYZ, I paid $10!!!!". I once bought a $5 Patagonia fleece from a thrift store too. But, I'd be surprised to find that online without some luck.
2) Cheap sleeping bag: I used a Campmor Hollofill sleeping bag in New Hampshire my first year of backpacking. And used it from Mem day weekend to Fall (October). It worked fine; if heavy and bulky. Of course I upgraded when I had the money!
3) No compass? That sounds alien to me. If you go off-trail, esp in areas a bit wooded, a compass helps immensely. I used it several times this past summer FWIW. I threw $10 out as a ball park figure. Even when I started backpacking, I had a compass on trail. Used to practice the skills I now have today.
4) Maps: Good point..but I did not included food prices either.
Free topos , and other maps, are online. They do require printing so that does require some money. Then again, I did not include the price of gas to get to the trail head either!
Maps, like food, are trip dependent, and vary in cost depending on what you buy, use and how much you need.
5) Sharing/borrowing gear: Absolutely. But, like the $5 Patagonia fleece, you can't count on that for every trip. Best to be independent if you want to get into backpacking in any way.
Keep the ideas coming as I've used them over the years with every revision.