What does "cost per bit" mean?

Solution 1:

The cost per bit is the monetary cost (i.e. in dollars and cents) per unit of memory, thats all, ie a way of saying "assuming the amount of memory required is the same". (You could equally say the cost per gig of memory - but mentioning the size implies a usage case. A bit is very arguably the smallest amount that can be stored)

Solution 2:

The cost per bit is just the price you have to pay for specific amount of memory.

If some amount of brand X memory costs $20 and the same amount of brand Y memory costs $25, then brand X has better cost per bit. The cost per bit differences are much more pronounced when comparing different kinds of memory (SRAM, DRAM, flash, SSD...)

Solution 3:

In addition to these other answers that rightly mention that cost per bit is the amount of money per bit:

If you have memory that stores 1024 bytes, that's 8192 bits. If that memory cost you $10 then it would be $10(cost)/8192 bits, or $0.001220703125 per bit. If you had memory with a faster access time, maybe it would cost $20 instead, in which case you'd have $0.00244140625 per bit.

Faster access time: $0.00244140625  cost per bit 
Greater capacity:   $0.001220703125 cost per bit