They'd be all plastic except for the swooping hawk head too (which unless I'm mistaken needs to be sawed off the body). You could save $30 per squad by just not using that head, and having a warrior head or something instead.
EDIT: If you wanted to go with more of a bat-wing scourge, a box of gargoyles would supply you with 10 sets of wings instead of 5. Then again, if people are going to buy a bunch of sanguinary guard and not use the marines, I'm sure there's no shortage of marine/BA players that could make use of the rest of the model.
EDIT (again): Actually, I haven't seen the picture of scourges in the codex at all, but Starcraft 1 makes me almost incapable of seeing scourges with anything other than bat wings. They also should explode if they assault a tank...