"How much does the vodka cost?"
Translation:Faint mae'r fodca yn costio?
Why not "Faint yw'r fodca yn costio?"....because yn costio makes it NOT a focused sentence.....yw only with focused sentences?
This is not to do with focussed or emphatic etc sentences.
Different question words (pwy, ble, pam, beth, sut, etc) are followed by either mae or ydy. In this case, where the question Faint...? is about how much something does something - costs or weighs, say - it is followed by mae*.