Text messaging packaging is starting to look like long distance used to, in some ways. In other ways, the packaging is starting to resemble voice usage packaging.
As per-message fees rise to the 20 cents a message rate for casual use, it makes sense to buy a bucket of minutes. That's a concept similar to the "presubscribed carrier" system for long distance calling in the U.S. wired telephony market. If users didn't pick a carrier, and use long distance on a "casual" basis, those users got socked with fairly expensive charges for used minutes.
Picking a carrier of record for long distance typically meant lower per-minute prices. With the rise of VoIP unlimited calling plans and "buckets of minutes" in the mobile market, that is less an issue than it used to be.
Text messaging, though, is moving that way. Carriers want users to upgrade to buckets, so pricing casual use at a high rate is one way to encourage that behavior.
In another sense, the new packaging also is similar to the voice bucket plans now used in the mobile industry. Usage above the bucketed amount incurs hefty additional usage fees. So people have incentive to buy bigger buckets than they think they will need as insurance against payment of those usage fees.
Text message pricing now has moved that way as well. The other effect is to increase the value of any unlimited texting plans.
One thing is clear enough: the highest revenue-per-bit service in the U.S. market is text messaging.