Haha, the really expensive, $150,000-$200,000 are pre-War (that's pre-WWII) Gibson original five-string flat head banjos. Why are they so expensive? First, Gibson did a horrible job documenting how it made banjos. The tone rings (a metal ring inside the banjo you can't really see) were made from a certain metal. That metal changed during WWII and never went back to that pre-war quality. Next, in the 1920s jazz was big. Gibson cut back on the number of five-strings it was producing, because a majority of the banjos out there were being used in jazz and jazz players used flat picks and wanted all but the 5th string. So Gibson produced a much larger number of 4-string "plectrum" banjos (it's a 5-string without the short 5th string). Finally, of the banjos being produced at the time, Gibson was pumping out more "arch top" banjos instead of "flat head."
Earl Scruggs played a pre-war Gibson 5-string. That's the sound people want in bluegrass.
Eventually the 5-string banjo became more popular, again, and the 4-string faded a bit. Today, one of those original banjos, unaltered are VERY hard to find.
What happens more often is someone buys a Gibson 4-string and has it converted to a 5-string (or purchase one that's already been converted). Those Gibson conversion banjos still pull in $5,000+.
In all honesty, the Gibson name adds another $1,000-$2,000 to the price of newer Gibson banjos (Gibson stopped making banjos a few years ago).
Now there are other good size builders such as Deering, Stelling, Gold Tone, Recording King, etc. and a number of small shops (including one man shops) such as Hatfield (my banjo), Yates, Sullivan, Nechville, Bishline, etc.
And you want to talk about people who are really into the tools as well as the music? Banjo players approach their instruments like a science.... trying to achieve the perfect set up.
I'm only a year into this, but I've learned so much. It's a fun, difficult instrument to learn. And my Hatfield is kind of like my baby. HA!