Cost... high or low? Is BSI expansive or cheaper to make? Sony for example has BSI on the lower end (RX100 III), than high-end (a7II/a7S). Regards
BSI has been more expensive...but costs are coming down. There are more risks to BSI. Your using both sides of the wafer, so that means there is twice the surface area within which defects can cost you a die. As ccs stated, the wafer is first "thinned", then more stuff is layered down on top of a thinner substrate. FSI (front-side illuminated) sensors tend to have a fairly thick silicon substrate which gives the sensor stability and rigidity. BSI sensors are more fragile, so even if you don't lose a die to a wafer defect, they can break during transport or integration.
Those risks, and the increased die losses per wafer, tend to increase the cost of BSI devices.
When sensors are small, like the 1/3" or smaller sized sensors (particularly the ultra tiny ones found in smartphones, tablets, other mobile consumer-grade devices), you can generate hundreds if not thousands per wafer, and their small size reduces the chance that the thinner substrate will result in fragility. A relatively large (gargantuan, compared to consumer device sensors) APS-C sized BSI sensor like the one in the Samsung NX1 carries with it a fairly high risk of either loss to wafer defects or loss to breaking. Samsung is a pretty advanced company, though, and they have a number of cutting edge patents for large-sensor BSI designs. They have techniques to strengthen the thinned substrate, they have ISOCELL technology (not used in the NX1, as far as I know, but it practically eliminates color crosstalk in Bayer type BSI sensors, which has been a problem in the past), etc. My guess is Samsung employed their substrate strengthening patents in their NX1 sensor, which is why they were able to create an APS-C sized one before everyone else.
Canon actually has patents for FF layered (not bayer) BSI sensors. They have for a couple of years. They haven't employed them yet, and my guess is they haven't found a way to make sensors that large (2.6x greater area than an APS-C sensor) stable enough to be integrated into a consumer grade DSLR.
Sony's BSI sensors are on the smaller side...a small fraction of the area of an APS-C sensor, for example.