You just translated the phrase directly from Japanese. Don't do it. Just say XXX is/ are rexpensive. Memorize it, and you will be happy!

"Expensive" describes the object itself; not its cost. "Cost" is related to "how much?", so you would describe the cost of something as either high or low. Despite that, you'll still find some sloppy examples where someone has written "the cost is expensive"... please don't copy such mistakes. ;)

Hi again Aki :) The word "expensive" is typically used to refer to the object, service, or thing being purchased. The cost itself can be said to be "exorbitant" but not "expensive." I know it might sound kind of weird, but it's just one of those unspoken rules in English. To make matters even worse, although "cost" is not usually described by the word "expensive", the word "pricing" which is synonymous with "cost", can be described as being "expensive."

It's not the way we commonly express that thought. Some common ways are: The cost is too high. It's too expensive. That item is too expensive. It costs too much. It's not a matter of whether or not you can say it. Rather, it's just a matter of what people customarily say.

Cost is the property of the object that describes its price. Expensive is the adjective of the object that denotes it as a high-cost object.