As to Dave’s comment: I'll bet that my iPhone has more power than our System-360/50, but it has nowhere near the sheer I/O throughput of a mainframe.

The ratio of bits of I/O to CPU MIPS  was very high back in those days, Particularly for the Model 50 which was considered a ‘Commercial machine’ vs a ‘Scientific machine’.  The machine was doing payroll and inventory management, high on I/O low on compute.  Much different today even for an iPhone.  The latest iPhone runs on a ARMv8 derivative of Apple's "Swift" dual core architecture called "Cyclone" and it runs at 1.3 GHz.  The Mod 50 ran at 2 MHz.  The ARMv8 is a 64 bit machine.  The Mod 50 was a 32 bit machine. The mod 50 had no cache (memory ran at .5 Mhz).   Depending on what instruction mix you want to use I would put the iPhone at  conservatively 1,500 times the Mod 50.  I might add, a typical Mod 50 system with I/O sold for $1M.

On the question of memory cost - this is from the bible on 360 mentioned earlier.

For example, the Model 50 main memory with a read-write cycle of 2 microseconds cost .8 cents per bit.  

Page 194 Chapter 4 ‘IBM’s 360 and Early 370 Systems”