For rear, legal, as dark as you want. You can hump inside also nobody can see if fully dark. However if too dark, you might have some issues to see at dark or dim area aka for reverse parking, etc.

I don't recommend fully black out. For me, VLT30 is dim enuf even under bright sunlight and yet gives sufficient dim lighting. If you want to go darker, VLT15 or 10, not lower in order not to impair rear view and are very dim to sleep in.

Pricing of tints can range from RM300-500 to RM1.5K for rear windows and windscreen depends on the brands and quality you choose.
Since you don't even tint the front, just the back for sleeping, I guess you would want low-mid end pricing tints.

I cannot recommended any of those no brands tints that goes for low hundreds.
Got no experience on low and mid-range tints so others might want to recommend. Can check out Totalgard for low and mid-end pricing with some quality assurance vs those no name or so-called made in USA, Korea or Japan cheap tints.