Learn Romanian: Rochia nouă este scumpă, dar cea elegantă este ieftină. - The new dress is expensive, but the elegant one is cheap.

Because cea replaces a feminine singular noun. Here, it stands in for rochia (the dress). If we were talking about a masculine noun like pantoful (the shoe), we would use cel.

Does cea just mean "one" in English?

In this context, it translates to "the ... one" (as in "the elegant one"). Words like cea or cel are demonstrative articles that point back to a noun we have already mentioned, so we do not have to repeat it.

Why do the adjectives nouă, scumpă, elegantă, and ieftină all end in -ă?

They all describe the same feminine singular noun, rochie (dress). In Romanian, adjectives must agree in gender and number with the noun they describe, so they all take their feminine singular forms here.

Wait, is the word for "dress" rochie or rochia?

The base word is rochie. The -a at the end is the definite article "the" attached to the noun. So rochie means "dress", and rochia means "the dress".