tp555: Haven't figured this out yet.
The (!) means that the price currently shown for that commodity has been specifically affected by the random event currently in effect at that station.
For some commodities, this will mean a higher price than would otherwise be in effect (Example: food items at a station under Famine). And sometimes it will mean a lower price than would otherwise be in effect (Example: Alien Artifacts at a station with an Archeolgical Find.)
Note that having the current price pushed up to "higher than would otherwise be the case" may not necessarily be enough event price inflation to push the price of a depressed commodity into the Green zone of "well above system historical median price" if the current price pressures at that station from other causes have pushed the non-event price particularly low. this is especially true at Impoverished stations where prices are generally a bit depressed.
So for example you may visit an impoverished station (or even a prosperous station that is in a long term economic downturn) that is nominally in the midst of a brief Economic Boom (which pushes virtually all prices upwards), but all of the price points are still in the red because the event bonus wasn't enough to bring the otherwise seriously depressed prices up to near of above system median.
For those who don't know, your best sell prices, across the widest number of commodities, occur during Economic Booms events, when a Korian Invasion is expected (NOT after it becomes a Korian Siege), and, especially, during a Militia Embargo. Fill the hold at a Market Glut, and run that blockade!
Trading with Traders & Merchants that you have rescued or just hailed in mid-space is also ALWAYS at excellently high prices, though for a limited number of units. (Sometimes mid-space trades with merchants is the ONLY way to get a decent price for Gold in particular. Gold prices seem to get depressed a lot, in certain systems. Of course, on a mid-space deal, you can't really dictate what commodity your trading partner is going to be interested in.)