How are storage costs computed for RMUs and FIUS?
It should be read as each unit costs that much. Something else to note is that once you hit 8 units you will actual pay less than you would for 7 units until you get up to 11 where it costs more than 7 units but is still cheaper per unit.

1 unit: $300 - $300 per unit
2 units: $600
3 units: $900
4 units: $1200
5 units: $1500
6 units: $1800
7 units: $2100
8 units: $1600 - $200 per unit
9 units: $1800
10 units: $2000
11 units: $2200