It was cheap compared to computers one would have otherwise run for timesharing, whether they were PDP-10, S/360, or some of the other contenders.

The real breakout for Unix was that it was something you could grab for free and later it was possible to port software to other platforms easier

That does indeed sound very cheap compared to the earlier time when there was infamously only a market for a couple of commercial computers in the whole world.

Enterprise gear today can pretty quickly and easily add up to seven digits per rack. At a previous job, I personally handled a specialized network processing card that retailed for over a quarter million dollars.

The PDP-11 seems like a bargain for what was fairly close to cutting edge technology at the time.

I'm not sure how to interpret these prices. Is your $100k number current dollars or 01973 dollars? Does it include terminals?

That price list seems to be mostly for the PDP-11/40, which according to http://gunkies.org/wiki/PDP-11_Memory_Management does seem to have supported a kernel/user mode split and a base register for keeping multiple processes resident in core, without any extra options, so I infer it was suitable for Unix. But I'm not sure if you might have needed the KT11-D addon memory management unit.

The 01974 CACM paper https://dl.acm.org/doi/pdf/10.1145/361011.361061 says Unix could run on hardware costing as little as [US]$40 000, which would be about US$250 000 today.

I think it's common for each office employee today to receive a US$2500 computer, so US$250k is a 100-employee departmental computer budget, not counting things like network infrastructure and servers. In other workplaces such as machine shops, coal mines, construction sites, and cattle ranches, capital investment per employee is commonly several times that.

Unix was not designed to support that many users—the uid was 8 bits—so the Programmer's Workbench users over the next few years took to sharing a single uid per a whole team of users. Later versions of Unix, of course, expanded the uid to 16 and later 32 bits.

https://ourworldindata.org/grapher/capital-intensity-vs-labo... seems to have a measure of capital intensity capital stock per worker, but I don't understand how to interpret it. The US today is around 198, and 103 in 01973, and the units are supposed to be inflation-adjusted (02010) dollars per work hour. But work hours are a flow, not a stock, and we're supposed to be measuring capital stock. So does that mean dollars per work hour per year? If so, that works out to about US$200k of capital stock per full-time worker in 01973, as an average across the economy. That would be a US$700k PDP-11-equivalent for every 4–8 workers in 01973, or for every 2–4 workers today.

https://fred.stlouisfed.org/series/RKNANPUSA666NRUG puts the total capital stock of the US at 23 trillion inflation-adjusted dollars in 01973, and https://fred.stlouisfed.org/series/PAYEMS puts total nonfarm employment at about 78 million for the country. This works out to about US$290 000 per employee, which is in the same ballpark as the French data but a bit higher, a US$700k PDP-11 for every 2–3 workers. But maybe a lot of the capital stock was on farms, which are excluded from the denominator here.