Not sure what the cheapest one is, I have an Iridium Extreme (I think it is higher on the price point). Ensure that you get one that is on the Iridium network, the other one is less reliable (or use to be, haven't used the other network in 5+ years because it didn't work well). The cost monthly is around $59 CAD and 1.50 per minute on-top. Another good option is satellite texting, it is much cheaper (SPOT, In-Reach seem to be good units).
For options you can take a look at www.roadpost.com
(I'm not affiliated, these are the guys I use)