What makes PC SDRAM so much more expensive than the same capacity in a chip?
To put the comments into the form of an answer so that the question can get closed...

The SDRAM part linked to is a 512Mx16 device, 512M addresses, 16 bits wide giving a total of 8Gbits.

The DIMM linked has a capacity of 8GBytes, 8 times the capacity of the individual memory chip.

The DIMM also lists its speed as PC1600 with timings of 10-10-10. For the memory chip the -PBA at the end indicates that it is PC1600 with timings of 11-11-11 (the PB) in a x16 configuration (the A).

So while the DIMM is 4 times the price you are getting 8 times the memory that can run 10% faster.