Presumably, the cost would be roughly the cost of traditional memory. In most consumer devices, memory is bottlenecked by monetary cost, not space or thermal constraints.

However, dedicate read-optimized memory would be instead of a comparable amount of general purpose memory, as data stored in one need not be stored in the other. The only increase in memory used would be what is necessary to account for fragmentation overhead when your actual usage ratio differs from what the architect assumed. Even then, the OS could use the more plentiful form of memory as swap-space for more in demand form (or, just have low priority memory regions used the less optimal form). This will open up a new and exciting class of resource management problems for kernel developers to eek out a few extra percentage points of performance.