A PRT rail line is only supposed to take up one lane, and could be put into a parking lane, so the real estate is fairly limited. It's also compatible with other traffic, unlike LRT or some other systems. Elevated heavy rail is much harder, because they are so heavy; as a result no one seems to be building them anymore. Those take up a lot of real estate, probably the biggest cost for doing any expansion (especially a dense city); going underground of course has its own very high costs. Anyway, potentially you wouldn't need anymore land, and SkyWeb Express is estimating 13-24 million $US in installation costs per mile. That's more than I've seen elsewhere, but seems pretty affordable in comparison to other systems.