"Delicious beef is very expensive."
Translation:맛있는 소고기는 많이 비싸요.
kinda has, kinda has not. The adverb '많이' is derivered from the adjective '많다', which means 'numerous'.
It is very common for Korean adverbs to be based on adjectives with '이' somehow attached at the end. For example: 열심하다 -> 열심히 (eager, eagerly); 영원하다 -> 영원히 (eternal, eternally); 빠르다 -> 빨리 (quick, quickly); 없다 -> 없이 (nonexistent, without)
however, DO NOT confuse this '이' with '이/가' subject marker! And also don't try to coin your own adverbs by adding '이' to random adjectives, because it usually isn't that simple. Better check the adverb you want to obtain in a dictionary.
Your sentence would be something like "As for deliciousness, beef is the most expensive." That's because using the topic marking particle after 맛있다 indicates that you are emphasizing the deliciousness of what you're about to talk about. "맛있는 소고기는 가장 빘아요" would be, "Delicious beef is the most expensive."
For distinguishing between 이/가 and 은/는, here is a video I really enjoy about this topic/subject (sorry for the pun!): https://www.youtube.com/watch?v=6JSJ9yEWy1I