>Walmart sells the card for $1, and Green Dot charges the usual associated fees: $5 a month if your balance is less than $1,000; $2.50 for ATM withdrawals; etc.

This is absolutely not how one should encourage people to save. This is Walmart trying to steal business from local banks/and or provide banking services to people in rural locations. Personally, I think Walmart should offer this at zero-cost to help their customers live with less financial vulnerability (and thus shop more at Walmart).

Full disclosure: I work at a startup that's using prize-linked savings to help encourage people to save money, but we don't charge a monthly-fee and we actually give our users interest on top of what they win.