Sand in Heavy Duty dry bags. Found the dry bags used for super cheap. I have a 25lb, 35lb and 40lb, to mix and match. $15 total investment and they are bullet proof. Best part is, they have handles and lashing points.
I add water bladders on my biggest and steepest days, then I can dump it at the top to help save my knees on the descent.
A friend that does a lot of ruking(sp) suggested bricks taped together with duct tape. Adjust weight with number of bricks. Packs well, durable and cheap.