The lowest cost molding materials are ComposiMold and Impressive Putty. Instead of purchasing a new molding material every time you make a new mold, you can re-melt and re-use ComposiMold and ImPRESSive Putty.

To make even lower cost, look for the USED ComposiMold. (https://amzn.to/3Ui7PlK) 

ComposiMold and Impressive Putty are the real lowest cost mold making materials available, and after you purchase it, you can make as many molds as you want without buying more.

With ComposiMold, you will need to use a mold box. Melt the ComposiMold and pour it over your object you want to duplicate. You can make simple molds, two part molds, and more complex molds and castings. 

Impressive Putty you don't need a mold box and you can even take indentations from ceilings or walls, or anywhere else. Just heat to soften the putty, and PRESS it over your object. You can also make two part molds with Impressive Putty.

A lot of the internet, suggests using a silicone caulking material for a cheap molding material, but this is actually more expensive than ComposiMold or Impressive Putty. The caulking material shrinks by up to 30% and does not last, and you cannot re-use it after you made your mold. 

1. Reusable Nature: ComposiMold is reusable, meaning you can melt it down and reuse it multiple times. This feature makes it cost-effective as you can create numerous molds with a single batch of material, reducing the need to constantly purchase new molding materials.
2. Ease of Use: ComposiMold is relatively easy to use compared to some other molding materials. Its simplicity means less waste due to mistakes, saving you money in the long run.
3. Non-toxic and Biodegradable: ComposiMold is non-toxic and biodegradable, which not only makes it safer to work with but also reduces environmental impact. Its eco-friendly properties may contribute to its lower cost compared to some synthetic materials.
4. Versatility: ComposiMold can be used for a wide range of molding applications, including casting various materials such as plaster, concrete, wax, soap, chocolate, and even low-melting-point metals. Its versatility means you can use it for multiple projects without needing to invest in different types of molding materials.