How much does this cost?
'Costa' — 'KOS-ta'. Short and clear. Don't confuse with 'costa' meaning coast — context is everything.
When the price tag is missing or you want to confirm. Perfectly polite in any shop — never embarrassing to ask.
'Quanto' (how much) + 'costa' (costs, from 'costare'). For plural items: 'Quanto costano questi?' Note 'costare' is a regular -are verb.
Quanto costano questi pantaloni?
How much do these trousers cost?
Plural subject needs 'costano'
Qual è il prezzo?
What is the price?
Slightly more formal way to ask
C'è un'etichetta?
Is there a price tag?
When you can't find the label
Italian shops often run seasonal sales ('saldi') in January and July by law. Discounts outside these periods are called 'promozioni' or 'offerte' and are worth asking about.