The SIM card is a tiny little disc that pushes in the unit. T mobile, att and a bunch of suppliers sell "unlocked" cards for like throw away cell phones. You can buy one of these cards much like a prepaid phone card, then renew as you need. Cards can be bought for $10 and plans 30-50 bucks.

How much data the unit uses is dependent on how frequently you have it report. So if you had it display location every 10sec it'll use a lot more data than if you did say daily or weekly.

Next time your at best buy or radio shack you can talk to them too. Best buy has a bunch of cards on their website. Hope that helps.