I'm not sure what the "units" are that your plan quotes. The vendors who sell Iridium products are bound by their contracts to sell at very similar prices. I got 500 minutes of airtime for about $750 US. It's good for one year, and you can be extended for a significant price. If you sign up for the same plan before yours expires, the minutes you have left roll over to the new plan. You can always add more minutes if you run out.
With the Iridium Go you can't talk (maybe through your cell phone ?). I use a phone with an added Iridium wifi gadget called AxcessPoint. It sets up a wifi hot spot. The laptop connects via wifi for downloading email and weather. It works like the GO, only charging minutes when data is actually downloading.
Doing it this way I can also do voice calls.