It seems anyone who can't fit a 15mm drive in their 2.5" bay can't go above 2TB unless they're willing to pay half a grand for a 4TB SSD. Arseholes. :@
From the brief searching I've done, I've seen suggestions that 2TB is possibly the biggest for 9.5mm anyway.
Thankfully SSDs are coming down in price quite quickly at the moment.
Because I've seen SSD prices dropping significantly across the last few months. Whether or not they're are the bottom yet, who knows, but they've definitely been going down more than 10%.