费 (fèi) has two related uses: as a noun it means a specific fee or charge (e.g., 学费 tuition fee), while 费用 (fèiyong) is the broader 'cost/expense'. As a verb it means 'to spend/consume (time, effort, money)', often in a somewhat wasteful or heavy sense — it's less neutral than 花费 (huāfèi) for spending money and more about using up resources, not about paying.