As for price, i don't think it will even take that long. In 2 years it will cost something like 50 bucks. A USB3.0 128GB flash drive (260MB/s read, 240MB/s write) from SanDisk now costs about $100.
E.g. on Amazon UK right now I see a 256GB Lexar SD card for 300 GBP, or a 128GB SanDisk SD card for 70 GBP, or 64GB SD cards for around 30 GBP...
So expect the prices on the 256GB models to start falling shortly, and once 1TB versions arrive it won't take long until the 512GB versions drop to a quarter or less.