The documentary is awesome and lots of fun (I think it's on Netflix now). I have a couple friends who are into modulars. It's not a cheap hobby! One friend seriously got started by winning on Jeopardy. There's some local folks I know too (Pittsburgh Modular), who make some nice starter eurorack kits.
And if you ever wanted to start building one (or just drool at other people's racks or wishlists), https://www.modulargrid.net/ is a very cool site. cEvin Key of Skinny Puppy has even designed and sells a few eurorack modules (as Subconscious Communications). Related note, I just saw SP live last month, and he had a huge modular rack with him. Intros and outros to songs were filled with crazy synth jams!
But, as you will see in the documentary, the first "commercial" modular synths started somewhere in the neighborhood of $10,000, and that's when a gallon of gas cost about 25ยข. Now that gas is $2.50/gallon you can get a very nice start on a modular for $1,000 (only 10% the cost of 50 years ago) and can add on modules for as little as $150. So, once you're started, you don't need to break the bank every time you want to add a new toy, and even the really cool ones are under four figures. It used to be you went with Moog, ARP or one of the custom shops, and they were not compatible with each other. Now there are dozens of little "custom" shops turning out really cool modules that are all inter-compatible via EuroRack and similar systems.
A few Moog Modular Systems:
(Well, of course I had to include my boyhood hero, Keith Emerson!)
ETA: I listened to that album, Walter, and a little something else of theirs. It's not my usual thing, but I kinda dig it. I enjoyed listening to it as I did other things. Thanks for the introduction.