So, how much is a PhD in CS from a well known and reputable institute really worth?

Usually, worse than nothing, except for three purposes:
1) You want to have a career in academia, which is a bad idea in general.
2) You are eyeing a management career and want a PhD for your business card.
3) You happen to have a challenging PhD topic that is highly sought in the industry by the time you finish the PhD.

Otherwise, it's:
1) A waste of time that you will lack in industry experience compared to those who go directly into the industry after studies.
2) You will lose a lot of money because the years where you work on your PhD will be paid much worse than a regular industry dev job.
3) If you can't get a job like mentioned in the previous enumeration, you will be regarded as "over-qualified", and the PhD will even hinder you finding a job.

Beats me, I've managed to get by without a degree.

Its great if you hate earning a lot of money

Ask yourself: am I fascinated by working on graph theories and / or machine learning? Am I willing to do everything it takes to become a valued researcher / leader at some large company? If the answer is no, then it's worthless.

Well I'm a Dev with no formal education in CS, senior Dev to be precise.

@Fast-Nop Came here to say exactly this.