Apple’s memory is always expensive to go to the higher config ($1200 for 64GB).
If you bought the 6-core Mac Pro as I did, then that 16GB trades in for $100 at OWC, effectively bringing the cost down to $795 for 64GB memory. Which in context of the system cost makes a lot of sense, though most users will find 32GB just fine*.
After you upgrade with OWC, save even more with OWC’s Trade-In Rebate for the original factory memory. Receive $75 for an Apple factory 12GB (4GB x 3) set, $100 for factory 16GB (4GB x 4) set removed from any Mac Pro 2013 model.
* If you want Lightroom to run lickety-split, get those catalog/preview folders onto a fast SSD, e.g. get at least the 512GB option, or add the OWC Accelsior PCIe SSD in the Helios enclosure via Thunderbolt, and host the catalogs there.