I would go to the manufacturers' web pages and see if you
can find a Packaging section in the technical stuff. I know
old-line outfits like TI, had published all their package
external drawings. But a FPGA package may likely have
been a custom development and whether it's got the same
level of documentation, would be hard to guess.
You could of course get a gram scale and a low-grade,
but same-package sample of each and weigh them yourself.
Distributor support engineers might be able to dig you up
package data.
You could take dimensions and materials and do a volume,
density based calculation - the big FPGA packages are
mostly solid, a bit of air around the die that you could
probably ignore. L*W*T*density(material) and you won't
be too far wrong (the tungsten on hermetics, or lead frame
in plastic, are second order I think). Ball array plastic might
have more weight in the bumps than the body if the bumps
are tin/lead and lots of 'em.