Is a PhD (economically) still worth doing? I used to think that with every degree you get your salary goes up.
Universities produce more and more PhD's and i learned in economics 101 that once the supply is higher than the demand, the price (here salary) will drop.

Even if you get a little bump in salary, you have to take into account, that you are joining the workforce 3-4 years later and therefore "loose" those salaries.

From an economical standpoint : Is a PhD still worth doing ?

( I know, doing a PhD is not "just" about the money )
If you look at income alone, it's definitely not worth it - assuming you don't live in a country in which "Herr Doktor" is still highly regarded, and an almost must for high-end jobs in politics and business (like in Germany or Austria).

In industry, there are a few jobs (chip design for Intel or quantitative hedge funds come to mind; maybe in consulting it's a toss up; certainly there are others) where it might help. There are also jobs where it's required to have a PhD (some chemistry research, or such), or at least very beneficial. But outside these somewhat rare positions, HR will put you on a pile called "non-standard degree" which is largely ignored, and you rely on personal connections or quantitative recruiters to even get your foot in. 

If you end up in academia as tenured faculty, you get a decent salary with a wide range (e.g., at the top end, Business School finance assistant professors lie around $200k at good schools, to a low end that's a fraction of that if the school or department is less wealthy, or the field less supported by industry). But if you're smart and driven enough to get there, you almost certainly could have long made partner at McKinsey by that time, where you won't go home with less than $1m. 

I think the term "still" in your title is a bit off. I don't think this ever was different. You should write a Ph.D. as you're passionate about a field, look for independence, and genuinely love what you're working on. And you should be aware early, and accept, that this is not a choice made for the money.