They are so close to 100% efficiency that the possible gains are modest.
I'm currently doing a conversion that uses a Netgain Hyper9 [1]. They cost about four and a half thousand dollars (including controller) and they're pretty bulky and heavy. They're really efficient though. An OEM manufacturer I'm sure could have a motor made much more cheaply and design it to run at a much higher voltage and produce correspondingly more power. I think the Hyper9 just uses magnetized iron or something like that for the permanent magnets. No rare-earths, so it might be pretty cheap to make something like that in volume. They serve the conversion market though, so it's kind of niche product. I have no idea what Nissan spends to make something like a Leaf motor.