It often refers to costs/expenses, so it can feel plural in English. But in Afrikaans, the form is does not help you decide, because is is used with both singular and plural subjects.
For example:
So whether koste is understood as cost or costs depends mostly on context and on the translation you were given.
Because Afrikaans and English are related Germanic languages, some words look very familiar. Is is one of them.
But the important grammar point is this: in Afrikaans, is is used much more broadly than English is.
English:
Afrikaans:
So is does not change according to the subject.
Duidelik is an adjective here, meaning something like clear, obvious, or plain depending on context.
In Die koste is duidelik, it comes after is and describes die koste.
So the structure is:
This is very similar to English:
Because in this sentence, duidelik is not directly in front of the noun. It is being used as a predicate adjective—an adjective that comes after a linking verb like is.
Compare:
But if you wanted to say the clear cost, Afrikaans would place the adjective before the noun:
So:
This is a very common learner question.
In Afrikaans, adjectives often have:
You usually use duidelik when the adjective comes after a linking verb:
You usually use duidelike when the adjective comes directly before the noun:
So in your sentence, duidelik is correct because it comes after is.
Yes. This is a very useful basic pattern in Afrikaans:
For example:
So Die koste is duidelik is a good model sentence to learn from.