It looks like there is about an $80 price different between the 4600 and iris pro chips at 2.3/2.4 speeds. I've never seen nvidia publish pricing for their mobile gpus but I can't imagine it would be more than 100-150 tops so you could be right. It wouldn't be worth it to price the two models within $100.
This is from Anandtech's Iris Pro review:
From talking to OEMs, NVIDIA seems to offer better performance at equivalent pricing with their GT 740M/750M solutions, which is why many PC OEMs have decided to go that route for their Haswell launch platforms. What Intel hopes however is that the power savings by going to a single 47W part will win over OEMs in the long run, after all, we are talking about notebooks here.
http://www.anandtech.com/show/6993/intel-iris-pro-5200-graphics-review-core-i74950hq-tested/19
So basically Iris Pro's only real advantage is power savings over a dedicated GPU. I don't think this would be very useful to Apple in the 15" since they already don't include usage of the dGPU in their battery life numbers.