It's so easy to forget that your salary is not everybody's salary. In Italy (a first world country afaik) a post doc makes a grand total of about 30 euros per day. An assistant professor probably around 45, which is similar to the entry salary of a software engineer. If you subtract food, housing and transportation, that $100 book starts looking very (very) expensive.
For a large fraction of the world's population time is not money...
If we assume the 30 euros a day was an average over the month, not the amount earned each work day, it's not all that different (30*30=900). Within 10% of what you stated at least.
True, but I was more focused on the fact that I earned that sum as a PhD, so one step below the seniority ladder. To earn exactly the same sum as a postdoc is just a crazy thing, and I'm glad I left to do my postdoc in the US.
anecdotally, I work with a lot of Italians and they tell me the salary for developers in Italy is around 2k per month, though that still seems really low for a western country to me.
In Spain, typical entry levels range from 18k€ to 25k€, so about 1200€/month, 14 payments, net of taxes (in Spain, as in other countries, the yearly tax you will pay is estimated and pre-taxed, and at the end of the year recalculated so the difference is adjusted paying more or returning to the contributor)
I was curious too : according to this NYT Op ed [1] " The average salary for an Italian born in the 1980s is about €1,000 a month, or about $1,375" Wolfram tells another story but I suspect the average and median wage are not useful in such a sample.
Let's not forget a free online book if posted on Hacker News is impossibly slow to access! I am getting 502's.
I am fairly new (3-4) years as a developer and I have gone the free route, still do. But Some of my best books came from early releases @ Oreily using the 50% off sales they almost always have floating around. Typically the free books are older and dated in my experience, but not always. Depends if you have the time to learn and possibly unlearn some things.
For a large fraction of the world's population time is not money...