There was a post on VK from someone who has analyzed what is more profitable: throwing items into the Cauldron or buying them in Shop, crafting them and then throwing into the Cauldron (if I understood correctly). He only posted the results. I have translated (that means copying into Google translate) the table:
From the comment section I understood that if you have a set amount of Coins it is more profitable to buy Iron sand and Wood, craft Shurikens and throw them into the Cauldron, than to just throw Wood and Iron sand directly into the Cauldron, but the resources will be random.
I don't quite know what numbers in columns 'Value' and 'Profit' mean.