To me the resources prices all follow the pricing rules of supply and demand. As more people sell something, they lower their asking price to beat others price. If no one is selling, the price goes up as people are willing to pay more.
Quick, guys, someone get a nobel price! 
Quick answer: Yes, the maps are weighted differently when it comes to resources. Both scenarios you describe are typical for that map.
So whats tyipical for 500p maps?
each map varies, but when those players which are adept in manipulating the stock market are present in map, you get huge average unit cost swings, look around players profiles on map that might help you determine the drastic swings you are seeing
