The author does lament the general trend of "pay secrecy" in part 2 [2] of his previous article on how he landed a job at Airbnb[1] but it's worthwhile to point out here. Almost any offer/salary does not exist in a vacuum but within the context of the current salary ranges (i.e. the market) for similar positions and also different positions.
Look at it this way: If I tell you that apples are 2 Simoleons (§) per lb, is that a good price? There's hardly any way to know, given that I quoted it in a fictional currency.
But if you know that oranges are 10§/lb, bananas are 5§/lb and that you can get a job at the grocery store selling them making 20§/hour, that gives some context to not only the prices of apples, but also the relative value of different produce and the ability to afford it. This is the essence of a market economy, which the job economy is an instance of.
Sites like Glassdoor and other efforts to share salary information[3] (even if they may be subject to reporting bias) are a good way to give you more information about the relative value of any offer, which will be another tool in your kit for negotiation. Read as much as you can, as in my experience it will be well worth your time.
1. http://haseebq.com/farewell-app-academy-hello-airbnb-part-i/
2. http://haseebq.com/farewell-app-academy-hello-airbnb-part-ii...
3. https://blog.step.com/2016/06/16/more-salaries-twitter-linke...