To be fair, we should be comparing similar service levels, i.e., unlimited talk, unlimited text, and 2-3GB of cellular data since that's what Straight Talk and Cricket offer.
Thus, Sprint would be $199 subsidized handset, $36 activation fee, $110/month for unlimited talk/text/data. Total cost of ownership over 24 months: $2875.
Verizon's costs would be identical to AT&T's.
Note that even if you went with the smallest voice plan on AT&T and Verizon and eliminated texting yet retained 2-3GB of cellular data, you'd still have a monthly cost of ownership of $79.79.
Here's a more complete table:
Monthly cost of ownership (over 24 months):
Straight Talk: $72.67 (unlimited talk/text, 2GB data)
Cricket: $75.83 (unlimited talk/text, 2.3GB data)
Verizon: $79.79 (450 min. talk, 2GB data, no text plan)
AT&T: $79.79 (450 min. talk, 3GB data, no text plan)
Sprint: $89.79 (450 min. talk, unlimited text/data)
Sprint: $119.79 (unlimited talk/text/data)
Verizon: $129.79 (unlimited talk/text, 2GB data)
AT&T: $129.79 (unlimited talk/text, 3GB data)
It still makes more sense to use Straight Talk since you're basically saving a few bucks off similar cellular data service from Verizon and AT&T, and they're basically tossing in unlimited talk and text.
Note that I'm only comparing currently available cellular plans, not something that someone has been grandfathered into.
That underestimates the difference for several reasons:
1. You can buy the iPhone unlocked (new) on eBay for less than the list price of $649. That would reduce the price of Cricket and Straight Talk
2. You're ignoring the overage charges. With AT&T and Verizon, as soon as you hit your data or minutes limit, the charges escalate quickly.
3. How many people do you know with a cell phone who never send or receive a text? You really need to add at least the base text plan for AT&T and Verizon.
4. There are a ton of extra charges on AT&T that it's easy to get hit with.
5. You can leave Straight Talk or Cricket at any time. Cancellation charges can be hundreds of dollars.