The noun (Substantiv/Nomen) Personalkosten is feminine in German. Therefore, the correct definite article in the nominative singular is die. So, you say: die Personalkosten.
personnel costs
Compound Words: The last word (base word) ALWAYS determines the gender. Most fundamental and reliable rule in German gender assignment This word is composed and its gender is always determined by the root word.
Die Gesamtheit der Kosten, die einem Unternehmen durch den Einsatz von Personal entstehen.
The total costs incurred by a company through the use of personnel.
Die Personalkosten sind in diesem Jahr gestiegen.
Personnel costs have increased this year.