The $181 million IRIS mission
(How do you read the expression above?)
1. The one hundred eighty-one dollar IRIS mission
2. The one hundred eighty-one dollars IRIS mission
3. The one hundred eighty-one dollar ai ar ai es mission
4. The one hundred eighty-one dollars ai ar ai es mission
None of those.
The one hundred eight-one million dollar IRIS mission.
Small correction:
The one hundred eighty-one million dollar IRIS mission.