According to Wikipedia, the patents for Moissanite expire in 2015. Does it mean that the Moissanite jewelry will become cheaper?
In most cases Patent expiry does not mean that things will get cheaper. Owner of IP doesnot reduce the price on its own it happens due to market pressure via competitor. if more number of players comes with same product at cheaper price then owner decides for lowering the price.
On separate note if patent lasts for its term that means patent was indeed strong and there are very good chances that owner might have some kind of IP portfolio to extend its market dominance.
With or without patents, polished Moissanite is very cheap, so while Moissanite by itself may become cheaper (see Pushpak's answer), jewelry prices won't be affected at all from this patent expiration.