I may be misremembering my source (an interview with the 8087 architect lead), but I want to say the die yield on the 8087 was like 30%... just barely feasible for Intel to actually make.

The original 8087 had low yields not only because it had a greater area, but also because it was made with a special technology that was used in few other devices (VMOS), so it was more complex and there was less manufacturing experience with it than with the technology used to make 8086 (HMOS).

Later revisions of 8087 used a standard technology and a shrunk die, with improved yields.