A pen is not as expensive as an article of clothing.
Pens are not as costly as clothes/clothing.
単数形にする場合
A pen is not as expensive as an article of clothing.
のように言えます。
as a clothes とは言えません。
as an article of clothing の代わりに as a piece of clothing と言っても良いです。
複数形にする場合
Pens are not as costly as clothes/clothing.
expensive の代わりに costly「高価な」を使っても言えます。
ご参考まで!