If using A:
Is THE dress made of silk cheaper than THE ONE made of cotton.
(THE=specific...talking about a specific dress)
B:
Is A dress made of silk cheaper than ONE made of cotton.
(A=general...In other words...
Are dresses made of silk cheaper than dresses made of cotton?
C:
Are DRESSES made of silk cheaper than ONES made of cotton?
D:
Is a dress made of silk cheaper than a DRESS made of cotton?
You can never use "a one" You must either use "a" or "one" It is like using the same word twice in a row...
a DRESS
one DRESS
NEVER
a one DRESS.