"What is the cost for advertising a product?"
Translation:Qual o custo para anunciar um produto?
Where I live (São Paulo), "anunciar" is common only in formal conversations (office talk). Informally, I hear far more often "fazer um anúncio/propaganda" ("ele fez um anúncio no jornal", "faço propaganda do meu produto"). "Anunciar" is absolutely more precise, for sure; but the common alternatives should be considered.
Questions using "qual" or "quais" with the verb "ser" ("é", "são") are slightly more formal. However, most of the time they are interchangeable, regardless of situation or formality. As a rule of thumb, omit the verb when you're feeling too lazy to speak much or are talking fast.
A few examples:
a) "Qual é o custo para anunciar o produto?" - People would speak this way in a business meeting.
b) "Qual o custo para anunciar o produto?" - People would speak this way in a business meeting, after a long streak of questions ("I want to know the duration of the ad, the time at which it will air, what is the cost...") or when feeling more relaxed.
c) "Quanto custa para anunciar o produto?" - People from the same company talking about the prices to advertise here or there. This is the standard way to ask it ("quanto custa"), give it preference in most daily conversations.
I think the above is an over-explanation, but I hope it can clarify more than confuse. Bear in mind that these formulae aren't final or definite, they are just broad directions. You won't necessarily sound unnatural by employing them in situations different than recommended.