As we have now obtained confirmation of the use of Ruby (+%experience) Gems does affect the paragon system. The importance of the helm socket has skyrocketed.Being the only slot where you can have a top-end piece of equipment and gain a bonus to experience gained in accordance to the Paragon system.
The gem prices are mainly determined by the crafting cost. Ppl with the plan will start mass crafting them and the price should get back to 20M or so in a few days. So if you don't have one yet I'd wait with the purchase a few days.