これはいくらですか。

Kore wa ikura desu ka.

How much is this?

Word for word

“As for this, how much?”

Politeness: Polite

です/ます style. The safe default with strangers, shop staff and colleagues.

When you would say it

Asking a price.

Worth knowing

Prices at shops are said in 円 (えん): 千円 is sen-en.