If the 1 TB drive in the new console is a HGST (Hitachi), the price difference between 500 GB and 1 TB is probably around 20-25 dollars.
A problem I see now is that HGST doesn't make a 2 TB 2.5" SATA hard drive, so they would have to use maybe a
Samsung which has a 2 TB drive for around $100-110. The difference between the 500 GB and 2 TB would be about 60 dollars or so. Sony may be exclusive with HGST, so that might not have been an option. There may also be some compatibility issues as well.
You're right though, they wanted to save money, but 2 TB is a better value since you're doubling your storage (1 to 2 TB) for an additional 30 dollars or so compared from the 500 GB to 1 TB.