On our last trip, not everyone could do the Verizon $10/day thing so we got the mifi and a sim card from mrsimcard.com

Had the same issue as bgs. Was doing ok and them boom. Done.

Biggest issue was that phones are set to backup data, pictures, apps etc when on wifi. On the Mifi, your phone thinks it's normal wifi so it backs up everything and transmits pictures at full size and it really tore into our data.

Be sure to turn off any photo to cloud back up along with App updates. Reduce the use of social media like snapchat, Instagram and Facebook. That should keep the data use to reasonable levels.

Also, hadn't thought about what aarpskier said about the International plan for the mifi. Will look into that for the next trip instead of a local sim card.