Решение:
To complete the sentence correctly we need to choose the correct form of the adjective "expensive" to compare the cost of a Mercedes and a Fiat.
In comparative forms of adjectives like "expensive," we use the "-er" ending for short adjectives, or we use "more" before the adjective for longer adjectives.
In this case, the correct form to use is "more expensive."
Therefore, the correct completion of the sentence should be:
"A Mercedes is more expensive than a Fiat."
This sentence compares the cost of a Mercedes to that of a Fiat, indicating that the Mercedes is more costly.