Things to consider regarding SIM cards
The information below is contributed "as is". There might be differences between operators. Always confirm all details yourself with the operator, and do not solely rely on this information.
Roaming - using the tracker outside the home mobile network.
"Roaming" is when a SIM card is used in a network which belongs to another mobile operator, typically when you are abroad. The traffic fees for roaming are often very high. Most trackers can turn off sending data when roaming. This is recommended.
If you need roaming, make sure you get a SIM card with reasonable costs for it.
Each time a device starts using GPRS a new GPRS session is started in the mobile network (don't confuse this with a TCP/IP socket). Operators typically charge a minimum fee for 0.1 MB data for a GPRS session. And a GPRS session is typically closed automatically if no data is sent for 3 to 5 minutes.
This means that tracking with a 10 minute interval can be very expensive, since each update creates a new GPRS session and you will be charged for 0.1 MB data for each session. But the actual data you sent is less then 0.0001 MB. This means that if the tracker needs to start a new GPRS session it typically costs you more, than all the data you send during one day if you kept one session.
If you have a vehicle mounted tracker where battery is no concern, it is recommended that you either use faster tracking interval, e.g. 1 minute, or enable keep alive features, which trackers like IntelliTrac and some TrackerOne implementations have. "Keep alive" will send data to the server with a tighter interval to prevent the mobile network to close your GPRS session.
Where battery is an issue, and you want rare updates, or updates on requests, SMS is often less expensive than GPRS. Since starting a new GPRS session often costs more than sending one SMS.
Flat rate / max rate
Many operators offer a maximum fee per day, or per month. This is very useful to control costs.
Operators may have different tariffs on different APNs. Always make sure you use the APN recommended by your operator.
SIM cards for telematics/data only
Some operators has SIM cards targeted for telematics. They often provide low cost for data traffic and SMS, and cannot be used for voice.
If you have feedback, information and real world experience about SIM cards and operators, please post it in this forum!