Hmm, I am thinking quite hard on this 64GB vs 256GB vs 512GB debacle since I will want to get my 12.9 ASAP. Here's my breakdown on the data I got based on my use case (iOS gaming, possible media editing or at least storage during travelling)
64 GB
Price: 1188 SGD (Singapore Dollars)
Price per GB: 18.5625 SGD per GB
256 GB
Price: 1338 SGD
Price per GB: 5.2266 SGD per GB
512 GB
Price: 1638 SGD
Price per GB: 3.1992 SGD per GB
Between 64 GB to 256 GB
Price difference: 150 SGD
Storage difference: 192 GB
Price per GB: 0.78125 SGD per GB
Between 256 GB to 512 GB
Price difference: 300 SGD
Storage difference: 256 GB
Price per GB: 0.85333 SGD per GB
Between 64 GB to 512 GB
Price difference: 450 SGD
Storage difference: 448 GB
Price per GB: 1.0046 SGD per GB
External Storage
1 TB 2.5 inch HDD: 70 to 80 SGD
120 GB / 128 GB SSD: 80 to 100 SGD
240 GB / 256 GB SSD: 140 to 200 SGD
512 GB SSD: 300 to 400 SGD
Storage Accessories
Apple Lightning to USB 3.0 USB Adapter: 58 SGD
Apple Lightning to USB 3.0 SD Card Adapter: 42 SGD
2.5 inch HDD/SSD USB 3.0 Enclosure: 30 to 50 SGD
Kingston MobileLite G3 : 40 to 60 SGD
Conclusion
Sounds like 256 GB may be better for me if I can set up my workflow such that photos and video can get worked on the iPad first, and then have them with their edits transferred into an external HDD somehow.
I did consider 64 GB seeing that my games took up 32 GB of space in my former 12.9 1st Gen 128 GB, but I have concerns about trying to sell it off when the next iPad comes. The Apple Store people also advised me against the 64 GB when they heard about how much space my games are taking up.
512 GB is the best when it comes to price per GB but I rather channel the 300 SGD to something else like the Apple Pen and thus expand capability of the iPad. Sure, I can always buy accessories later, but being able to save 300 SGD from something that I don't potentially need is equally tempting too.