Step 1: Identify the idiom.
The idiom is "costs an arm and a leg", describing the price of a plot of land in the sentence.
Step 2: Understand its figurative meaning.
This idiom means something costs an extremely high price, so high that it is as if you are giving away your own limbs to pay for it.
Step 3: Match the correct option.
"Very expensive" is the correct interpretation. "Moderate", "cheap", and "for sale" do not capture the meaning of this idiom.
\[ \boxed{\text{very expensive}} \]