Sheep is the sheep skin. To make it soft, flowing, like a fabric (well, not exactly cloth), the skin was made in a special way for a long time. There were a lot of technologies. But one of them provided for salt treatment. For one skins, it was required about 1.5-2 kg of this raw material. Salt in ancient times was very expensive, it was valued literally in gold.
http://svetich.info/publikacii/apk-respublika-kazahstan/kazahstan-ovchinka-vydelki-ne-stoit.html
Another version of the origin of this expression - for the skin to be soft, it needs to be properly cleaned and thoroughly washed. People who were engaged in this trade, called kozhemyakami. The work was very hard. Imagine what it's like to work with a wet skin, which needs to be washed more than once, and then dried.
Sheepskin coats, hats, mittens, and shoes were sewn from such a sheepskin. The better the skin, the more you can earn on it. By the principle of fabric - there are expensive (thin) cloths, there are thick (cheap).
And now let's think - if the sheep's skin (that is, the sheepskin) is bad, not of high quality, is it worth spending so much money and effort on its dressing? Will they pay off in the end?
Therefore, the expression "a candle is not worth it" means:
- the result is not worth the effort;
- unprofitable business, unprofitable business.
This expression is often used today.