i dont think you need to go to coconut fibre. they can be quite uncomfortable. i suggest buying a bed from one of the known brands that state it is for back-care. i bought a vono bed and its been great. it is harder in the middle of the bed than the top and bottom part so it gives support to your hips and comfort to your upper body and legs. the difference isnt obvious, you just feel supported properly.

What's the average price for coconut mattress?

RM800 for King size is darn cheap. Compared to those branded latex easily go up to 5k-8k

Even non branded or not fully latex also atleast 1K isnt it? For eg Vono

Thank you for the enormous input. I used all the info I got from you all to goreng the selesman at the gallary hahaha.

So far I've tried

Normal foam - cheopo soft. I think not good for the spine. Won't last long obviously.
Latex - Comfortable, soft, but did not 'force' my posutre to a correct one imo.
'High-density foam spring' - Quite expensive than the others. Comfortable like hotel one. For spine wise, hmm so-so i guess.
Coconut fibre - Wow..... hard like lying on the floor. I can see myself to get used to it, but at the moment....wow...dem hard hahaha. The pro is my spine is 'forced' to be on a correct position.
High density foam - This is best of both world. Stiff enough to make my spine into correct position, but not that hard like lying on the floor.

I think its either coconut fibre or high density foam. Both from the Mastercoil brand. Just need some time to rethink 

*btw I am a 100% back sleeper.