Is "I bought it because it costed ..." an idiomatic and grammatical phrase?

Question

I thought the phrase was common. Example sentence:

Naomi bought the coffee because it costed ¥499, and she was obsessed with prime numbers.

However, Google only yields 4 results. This makes me suspect there's something wrong with the phrase.

Answer 1

The reason you found so few results is most likely because cost is an irregular verb. The past simple (and the past participle, too) of cost is still cost, so the correct phrase would be:

Naomi bought the coffee because it cost ¥499 [...]

You can find many lists of these verbs on the Internet.