here in europe you can calculate (with realy very very good contracts) 2 Euro (=ca. 2 USD) per month per device incl. european roaming.
The main problem (i know from many countries) is the validity of simcards (if prepaid) and no avaiable low cost postpaid sims.
if you dont use special m2m sims its getting complicated.
Another problem is the billing increments of data. For us a 1KByte increment is very good, 10KB is already bad (but most european providers bill in 100KByte increments or for example Austria in 1MByte increments).
If you plan to use prepaid simcards have in mind, that you need to check the balance automaticaly (and handle all changes what your provider is changing on their website), handle automatic recharging of the prepaid sims etc.
And if you get a few thousand prepaid sims, you will really love to have a software which automaticaly scans all your prepaid sims...
(and if your provider suddenly block extensive requesting for data, for example block your fixed IP for 24 hours if you scan more than 20 sim), you are really f*cked up....
Politeness dictates it to write his name on a post