Three cats, Karachi, and I have tried all of these for at least two months each. Costs are per month for three cats — divide by three for one.
Imported clumping: Rs 9,000–11,000/month. Best product by a distance and I stopped because I could not keep justifying it.
Local bentonite clumping: Rs 4,500/month. This is what we use. Clumps well enough, dusty on pouring, controls smell for about two days between full changes. The dust is the real drawback and I now pour it slowly and away from my face.
Silica crystals: roughly Rs 5,400/month. Excellent on odour, but two of my three disliked walking on it and one protested on a rug to make the point. If your cat accepts it, it is good.
Sand: effectively free. Heavy, tracks everywhere, no odour control at all so it needs changing daily, and it is hard on a litter box. Fine as a backup, unpleasant as a permanent solution.
Sawdust / wood pellets: Rs 1,200/month and this genuinely surprised me. Pellets from a timber shop, not treated wood — you must check it is untreated, chemically treated sawdust is not safe. It absorbs well, smells of wood rather than of cat, and it turns to sawdust when wet so you sift the sawdust out and top up. My only complaint is it does not clump, so you have to change the whole tray more often.
Shredded paper: does not work. Goes to papier-mâché, smells within a day.
Ash — and I want to underline what you said in the post. Please, no. It is alkaline and it burns paw pads over time, and cats groom their paws so they eat it. The dust is bad for their lungs and yours. It is still the most commonly recommended thing in this country and it is genuinely harmful.
On switching, because this is where people fail: never swap in one go. Mix the new into the old, quarter, then half, then three-quarters, over a fortnight. A cat that finds an unfamiliar substrate in the tray does not think "new litter", it thinks "that is not a toilet" and goes on your bed. And if you change litter and move the tray in the same week, that is entirely on you.
One more thing worth more than any of this: one tray per cat, plus one. Three cats, four trays. Almost every "my cat has started going outside the box" post is a litter box availability or cleanliness problem, not a behaviour problem.