They are different chipset, so hard to compare them directly.
X99 will allow for the i7-5820k as the minimum 'gaming friendly' chip. That is about $300, DDR4 memory, most common config being 4x4GB, Close to $200.
Z97 takes anything from a lowly pentium up through an i7-4790k as well as the upcoming Broadwell chips. Boards are cheaper, memory is cheaper, and you don't have to shell out for an i7 unless you really want one.