Must Finish Trebuchet!

So I have a fairly heavy problem on my mind, its about (iirc) 16in wide, 3ft High (at the very top max) and 3-4ft wide. I need a solution like yesterday for the trebuchet counterwieght, I need it fast and I need it cheap (like maybe we can manage $100 MAYBE). That is as you can probably tell a fairly bad combination. :P

I'm trying to complete this project before marshall leaves and everything looks to be lining up nicely, except for this. My current ideas for this problem involve somehow getting various scraps of metal. Including (but not limited to)

-Big Hunks of steel
-Disposed Train wheels (where?)
-Discarded large flywheels (or lots of little ones?)
-600lbs of machining filings (how to contain them?)
-Discarded big rig brakes

-Earthen Materials
-water (containing 600lbs of water moving at x mph is no easy thing)
-sand/dirt
-concrete (the easiest but also the one my team is most opposed to)

-Dangerous things
-600lb of lead acid batteries (gotta be against the law)

Then after I figure out what material and how to mount it I have to figure out how to transport it. That used to be an easy problem, but now that i lack a running pickup its not nearly so trivial. :/

Any ANY input and suggestions would be greatly welcomed.
I will try to keep a more open mind than my other teammates, if your solution has enough merit I will try to pitch it to them. :P

If the material is sufficiently cheap I may even try to weld up a box to contain it (in this kind of shape) .

Coming soon to this blog: :P
Unfufilled Potential: How computers have let us down....
I want credit for this: An idea I have long kept to myself that may soon be possible and I want to be noted as a creator so I will release it publicly...finally