On the plus side, Sprint can afford to offer cheap rate plans on the VMo iPhone because
1. They aren't subsidizing the phone. At all. Probably making money on it, in fact.
2. They don't have to pay anyone for roaming charges, since VMo phones don't roam.
So they're willing to offer a $30 plan of some sort (which has a reasonable amount of minutes if you mostly use messaging/data), and have unlimited available for $50, $5 more than T-Mobile or AT&T unlimited via Straight Talk. The plus of Virgin Mobile here is that you don't have to find an Apple store and order the SIM-only kit to get your prepaid iPhone; it's all sold in one (well, many...Sprint stores everywhere) place.
The plus against CricKet? More coverage, a slightly cheaper, slightly more generous-with-data-before-throttling unlimited plan, and a 1200-minute plan that's a full $15 per month less expensive than CricKet's only iPhone plan. If you only talk around a thousand minutes per month (which is way above where I sit on my Sprint account, even counting every single minute), the $40 plan plus the non-subsidized iPhone would end up saving you money vs. CricKet after month 10 or so.
Heck, if you don't mind not being able to roam, you could compare the 1200 minute plan to Sprint's 450-minute Everything Data plan. Total cost: $80 per month. Despite a steep $450 subsidy on the device, you'd come out ahead on Virgin Mobile after 12 months, assuming again that you're averaging 40 minutes or fewer on the phone each day, and use less than 2.5GB of data in a month.
I bet Sprint sells more of these phones than they think they will...