what are the Cheap substitute of menthol that can be used in mint?

There are absolutely no substitutes, neither cheap nor expensive.

First, the cooling effect is due to a very rare coincidence. It so happens that menthol is chemically capable of activating one of the temperature receptors in human skin (also present in the lining of the mouth). There are no other substances which do the same thing, at least not ones known in cooking (and if there happens to be some exotic option, it will certainly be more expensive to get hold of).

Also, even if you could mimic the cooling effect of menthol, you would never find a substitute which also smells of menthol, so people will readily notice the difference.

That being said, I am surprised that you find menthol very expensive. I found that I can get natural mint essential oil (which is about 50% menthol) for 8 Euros for a 40 ml bottle. This should be enough to aromatize about 400 kg of candy. And we are talking about expensive, naturally created stuff sold to hobbyists in a country with high standard of living. It could be possible to find synthetic menthol at cheaper rates somewhere, most likely at a pharmacy. Of course, it could turn out that there is no good supply where you live and the few people who sell it demand too much, but this would surprise me if it is a commonly used ingredient in your culinary tradition.

If you find that pure menthol is for some reason too expensive for your pocket, e.g. because the smallest batch you can buy is more than you will use up in a lifetime, you could just look at products meant for cooks which are basically diluted menthol. This includes mint extracts, mint syrups, mint essential oil, and a lot of other options, varying by region.

There is a known cooling perception effect from solid coconut fat melting, and such is used occassionally in commercial confectionery - but this is a different effect from menthol.