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

2 answers

None of those.

The one hundred eight-one million dollar IRIS mission.
Small correction:

The one hundred eighty-one million dollar IRIS mission.