…こそ…が

but; although (emphasis)

JLPT

Structure

N + こそ…が
Na + でこそあるが
V-ます + こそするが

Meaning

This pattern is used in the form [XはYこそ…], acknowledging "when it comes to X, it's indeed Y", but then adding that there is still a contrasting state to what has just been acknowledged. Followed by conjunctions like […が], […けれども] (but). Used in writing.

このかばんは

、

値段ねだん

こそ

安やす

いが

、

とても

丈夫じょうぶ

だ

。

This bag is cheap, but very durable.