I guess your don't have to pre-pay for simbiotes, but then just don't forget you have to pay ;-)
Bone Density Augmentation: "Your Body attribute is increased by the bone density Rating for the purpose of damage resistance tests"(Core 459). It'd odd language but I believe it is intended to just modify damage resistance tests, not actually increase the attribute. The pricing indicates this for me, as other bioware that increase attributes (Muscle Toner, Cerebellum Booster, Cerebral Booster, etc) are priced at 30k+ per rating as opposed to 5k for bone density. Is there another place you are getting this from than the core rule book? I could have missed an errata.