I put "How much money for one watermelon?" but it's wrong. Feels like it should be correct.

I answered "How much for a watermelon"- is there a difference?

Do people in China eat a lot of watermelons?

When do we use "一个" and when "些"?

How much is the watermelon should be fine, too. I know it specifies "one" in the Chinese, but in English, this would be an appropriate version of the translation.