It's more expensive. It's less cheap.

[Mori]

Can we use "less" with short adjectives?

We use less with longer adjectives (interesting, beautiful, complicated), but we don’t normally use less with short adjectives of one syllable (big, good, high, small). Instead we use not as … as …, or not so … as … Not as is more common than not so:
This new laptop is not as fast as my old one. I’m sorry I bought it now. (preferred to is less fast than my old one.)
Ronald Carter et al., English Grammar Today, p. 132

We use less with both long and short adjectives.
It's more expensive. It's less cheap.
John Eastwood, Oxford Learner's Grammar: Grammar Finder, p. 287

 

[Tarheel]

I would not say "less cheap".

 

[SoothingDave]

"Cheaper" or "not as cheap."

 

[Mori]

"Cheaper" or "not as cheap."

Can we say this grammar point is kind of idiosyncratic? Doesn't the choice depend on the context, register, and intended meaning?

 

[Tarheel]

No, it's not idiosyncratic.

 

[jutfrank]

Can we use "less" with short adjectives?

The short answer is yes, but we do often tend to avoid it.