I would just cheat... like make a memblock mesh from a combined copy of the object - like make a mesh from it, then a memblock, then work out the memblock size, -12, then divide by 36.
Or, lock the vertex data and check the polygon count in that.
If you do this, perhaps your just as well adding more checks, like the mass of the object, maybe the coverage of it's texture, as performance would be affected by texture resolution as well as poly count. For example, if you have 2 cube objects, one of then unwrapped so each side uses a different part of the texture, and one that uses the same texture on each side - you could say that only half the sides are visible most of the time, so work out the total spread of the texture. A texture on each side means a texture size factor of 3 (texture width x 3), and the unwrapped cube would be a texture size factor of 0.5. In the long run it might help with optimizations.
Health, Ammo, and bacon and eggs!
