$5K is for the base model (8 core 16Gb I think) - $16K is the fully loaded 18 core version with maxed out storage 4 TB SSD, 128 GB RAM, Top GPU etc.

Traditionally with Mac Pros they would really gouge you on the price of extra RAM and storage - so what you would do is buy the the base model with the CPU you wanted (since the CPUs were actually good value) and then buy and add the extra bits in later.

Obviously there is no option to do that here so you really pay the premium for the extras.