I found why on my mod+vanilla at least, the supplies price were broken:
The refinery was a bottleneck.
On my mod I was trying to fix all raw materials, then fixing other stuff... after I noticed some wrong numbers when accounting only supply/demand, I decided to also look for stock/demand
The results were "eye opening"
when supplies have price settling between 100 and 200:
volatiles had a 1.6 stock/demand (probably because of the "depots" making the supply/demand calculation wrong)
organics has both 1.1 stock/demand and 1.1 supply/demand
ore, AFTER I modded the mines, was 1.5 stock/demand, 1.0 supply/demand
rare_ore, AFTER I modded the mines, was 2 stock/demand, 0.6 supply/demand
metal was 0.8 stock/demand 1.0 supply/demand
rare_metal was 0.4 stock/demand 0.6 supply/demand
the only source of rare metal in the game is refineries, thus it was clear something was wrong with them...
I tried to apply my usual code of production based on profit, the result was even more bizarre, my debug printed that the refinery profit making metal was 200k credits, but the profit making was negative 50k credits...
So I made a spreadsheet to facilitate things, and start fiddling with number, reached a result I wanted, pasted in the game...
And hooooray, supply price dropped to average 40!!!
Still, it was supposed to be 100...
So I will fiddle more until I fix it... still, I found interesting that the reason why supply price was broken was the metal refinery...
EDIT:
more fiddling with the refinery, and some minor fiddling with mines (they were making by default too little rare_ore)
also I had to edit slightly the rare ore and rare metal prices.
for now things are going well, I will need to tweak them more later (they are producing until stock/demand hits 3, then the profit system kicks in and they start to produce at 40% of capacity, then market gets stable that way... I would preferred if instead the stock was about 1.5 and production was higher).
But for now I am happy.
Also fuel and supplies on jangala are now averaging the base price, sometimes it go lower, hooray for new players
Now from testing around jangala at least, what needs fixing is domestic goods (jangala is undersupplied with them, since I have no idea what makes domestic goods, and how, I can't even start figuring why yet).