Learn Icelandic: Kostnaðurinn ykkar er ekki mikill.

In Icelandic, when you use possessive words like ykkar (your, plural) and okkar (our), the noun they belong to almost always takes the definite article (the 'the' suffix). So you are literally saying 'the cost of you'.

Does the word ykkar change to match masculine, feminine, or neuter nouns?

No, it doesn't! Unlike singular possessives like minn (my) or þinn (your), the plural ones like ykkar (your) and okkar (our) never change their endings. They stay exactly the same no matter the gender, number, or case of the noun.

Why is the word for 'much' here mikill and not mikið?

The adjective has to match the noun it describes. Kostnaður (cost) is a masculine noun in the nominative (subject) case, so we use the masculine form mikill. You often see mikið, but that is the neuter form, which is also used as a general adverb.

Can I use ykkar if I am speaking to just one person?

No, ykkar is strictly plural, meaning 'your' when talking to two or more people. If you are telling one person that their cost isn't much, you would use the singular possessive: Kostnaðurinn þinn er ekki mikill.