I have heard it in a different usage - "It will cost me dearly." Dearly is an adverb describing the cost (adverbs end with -ly). When used as "dear" in the context of "That is a dear coat," it can have a different connotation, and is more commonly used to describe something that is admirable as well as valuable (e.g., She is such a dear.). English is awash with words with different meanings and connotations depending not only on the context, but also the region. It ia no wonder that translations can be confusing!
"Dear" meaning "high in price" is marked as "generally dated" according to Wiktionary (https://en.wiktionary.org/wiki/dear#Adjective). While I'm familiar with this meaning of "dear", if someone said "It is dear" I would think the thing that person is talking about is a cherished object as this is how "dear" is most commonly used nowadays.
I agree that the adverb "dearly" is more common in its usage with "cost" ("It cost me dearly").
After це a dash can be put, if you want to stress the predicate and its meaning in the sentence, for example: Це - моя мати. In this case, a pause is released with intonation.
If you do not want to emphasize the value of the predicate, you do not put a dash after це.