Hi guys - I'm sorry I missed this thread and didn't respond sooner. This is Seth @ IGS.
Most all of the prices in the gem price guide were updated between 2013 and 2014. It is true that many of those price matrices have not materially changed since then. The price guide should be used as a proxy... the reality is that it really is just a "guide" There are so many factors that govern the price of a gem (most especially color, clarity, and general appeal) that they can't ever be quantified in a guide. The objective here is just to give you some general parameters.
All that being said, I am currently in the process of completely re-working and updating the gem price guide to make it more accurate, useful and updated. During that process I intend to bring several of the most significant prices more current. This is quite an undertaking as it requires many conversations with dealers and primary market participants.
In the meantime, if you are a paying member and you want some advice on a gem price, I'll try to help you out over email in a reasonably limited fashion. Just email me at firstname.lastname@example.org.