These are the 512GB and 1TB SSD upgrades for the 2015 MacBook Air, iMac, Mac Pro, Mac mini, and MacBook Pro. The rebate has been up to $200 in some cases, so upgrades like the Mac Pro to 1TB SSD have gone from $800 to $600.
The equivalent in euros for the Spanish store for the same extension is 720 euros, so despite the difference it does seem that a similar reduction has been applied . In Mexico the expansion now costs 13,800 pesos. The base configurations are still the same price.
The exact reason will never be known, although it is quite likely that the rebate was applied simply because SSD storage is becoming cheaper and more common in the market. And although the vast majority of users don’t need 512 GB of storage or settle for an external hard disk, rebates are welcome.