```python
class TieredCostCalculator:
    def __init__(self,:        pass

    
    def tier_cost(self, units: int, allowance:        if units < 0:
            raise ValueError("Number of units cannot be less than zero")
        
        if units <= allowance:
            return units * first_rate
        
        return allowance * first_rate + (units - allowance) * later_rate
```