The best and most economical Filler is Kiln Dried Sand
, available at most building supply stores. It is important that it is DRY sand.
The drawback to sand is that it can contain very fine power. If your stands are not 100% seal on the bottom or sides, sand dust can leak out. Also, there is a slim chance that the Sand will act Hydophilic, that is the sand will draw water out of the air. One potential solution to both the Hydrophilic aspect and the Dust, it to place smaller amounts of sand into Zip-Lock bags, and drop the bag into the stands until it is full or until it reaches the weight you desire.
The next step up is Aquarium Gravel
. Very fine but not dusty. A bit more expensive but far less than the Atacama fillers. Aquarium Gravel is also pH neutral.
You can get all kinds of Aquarium Gravel in many bright colors, but the cheapest is probably straight forward fine rock gravel -
Most Walmart Store have basic Aquarium Gravel in stock in the store in Pet Supplies. And of course most general Pet Supply stores will have Aquarium Gravel.
As to lead, keep in mind that lead is toxic. It might be cheap enough, but in the future, much like asbestos, you are going to have a hard time getting rid of the lead. Even BB's are just copper coated lead, so no real advantage there. Plus, don't expect 50 pounds of BB's to be cheap. Every year they become more strict about Lead, soon fishing weights and bullets/shot will no longer be lead. I foresee a time in the future where 50 to 100 pounds of lead is going to be very difficult to get rid of.
Yes, you can probably use small Ball Bearings, but don't expect 50 pounds of those to be cheap.
Last alternative, if you know someone who works in a Metal Shop where they are punching holes in metal all day, you might be able to buy those Punch Outs by weight for the price of scrap steel.