Unsmoothed natural walls actually are worse than block walls if both are of ordinary stone, while smoothed natural walls are just as valuable. It's engravings that really push the value up; an engraving is worth 10*the material's modifier*the engraving's quality modifier, which is added to the smoothed wall's value.
From the wiki's
article on rooms, which includes a section on room value, a rough normal-stone wall is worth 1, while a smoothed normal-stone wall is worth 5. 2-value stone (like flux and cheap metal ores) are 2 for rough and 18 for smooth, while 3-value stone (obsidian) is 3 and 27. Rough constructed stone walls are worth 3*the stone's material modifier, while block walls are worth 5* the stone's material modifier.
All this taken together means that gold and steel floors and walls are better for value than normal, 1-value stone engravings will ever be. Flux should never be covered by anything less than aluminum and platinum, while obsidian walls and floors are worth more than anything save adamantine. High-value soaps can outdo anything, but soap is of course very difficult to get ahold of in large quantities (even somewhat harder than aluminum), so is usually ignorable.
Since the materials needed to outdo even common-stone engravings are so rare, it's almost always better to keep natural walls and floors than to try constructing rooms.