I have exactly the same issue. The prices have been stuck since November 15 2015 - that seems to be the last time that the prices updated. (I took a long break from playing this game which is why I didn't notice it for so long.) I know the date when the wheels fell of the wagon because I could see 1 month, but not 1 week and not 1 day, until December 16. At this point, the prices disappeared. As you have noticed, the prices are the same in-game. It is not simply the Global Market miniprogram.
I'm lucky that the prices happened to end at a particularly good point. The price of oil is absolutely terrible ($8500/ kbarrel), so my OmegaCo cities are tearing it up. However, I would like things to change because the price of alloy is also poor, and I'd like to run one of my foundry cities.
I know that it's not my computer, since I have Origin installed on three different computers, and each one shows the same problem. I also know that it is not my account, because my wife has a completely separate account with Simcity: Cities of Tomorrow, and she sees the same stuck prices that I do, although she uses the same computers that I do. For that matter, we don't seem to have a problem playing multiplayer maps. Our cities can communicate and interact the same as they always have. It seems to be JUST the global market that is non-functional.
This is the first that someone else has this problem. Whenever I try googling "Broken Global Market", the vast majority of the results concerned launch issues from two years ago.