Even that isn't really the reason as to why it's cheaper... That's moreso the reason as to why AMD's marketting team didnt make it more expensive.
But it is cheaper because it is cheaper to produce. Intel has much more overhead, running their own foundry, using 22nm lithography, with varying types of transistors. CPUs most be priced higher in order to make up for this overhead.
AMD gets their CPUs from Global Foundries; their lowest lithography is about 28nm IIRC, and they do have plenty varieties of transistors, like Intel. However, their only source of income is not AMD, so their overhead doesnt need to be covered by just one company, so they can sell their fabrications for less. In turn, AMD can also sell these for less and still reap lots of profit.