Based on contract prices. Someone said their Sprint bill was roughly half that of AT&T. I'm not sure about T-Mobile.
Contract prices are slightly lower on T-Mobile (I pay for 3G but do not get it on the iPhone), although some of their prices have recently gone up. The effect is more pronounced for older contract holders. If I compare my contract to AT&T published prices...
Me = 1000 min @39.99, 400 SMS @4.99, Internet @ 19.99 = 64.97
The closest currently published T-Mobile plan (for comparison) is...
T-Mobile = 1000 min @39.99, Internet @24.99, 300 SMS @4.99 = 69.97
The closest published AT&T plan is...
AT&T = 900 min @59.99, 200 SMS @$5, data @$30 = 94.99
I think? It was a little hard to find a non-unlimited SMS plan on the T-Mobile site -- the price I put in above for 300 SMS is quoted when I searched for text messaging, but it didn't come up in the options when I tried to price out a G1 + plan. If unlimited messaging were done instead, then for the published T-Mobile plan, the internet rate jumps from $25 to $35 (but the $5 for SMS gets removed, so unlimited msging costs $5 additional to what's listed above) and for AT&T the messaging plan cost jumps from $5 to $20, so the published price difference becomes T-Mobile = $74.98 and AT&T = $109.99.
This is
without MyFaves on the T-Mobile side; T-Mobile includes wireless hotspots on any device (I'm not sure if AT&T finally does that now, or if it's still officially only the iPhone) and AT&T includes rollover minutes, which T-Mobile does not.
This is also exclusive of any kind of corporate or FAN discounts. And it's exclusive of fees (I think fees may magnify the difference slightly, since I think some but not all fees scale slightly with the total monthly costs).
Of course this is also exclusive of all kinds of other differences like how many hotspots AT&T maintains vs. T-Mobile, how big the relative 3G coverage areas are, etc, etc.
So, from these calculations, based on the published rates for T-Mobile (not Sprint) and AT&T, at roughly comparable levels of plan feature, the T-Mobile plan cost is 74% of AT&T assuming limited messaging and 68% of AT&T assuming unlimited messaging.
I think if anyone is arriving at "half the cost," they are comparing a corporate discount / FAN type of plan on one network to a standard published rate plan on the other network, though.
EDIT: I don't completely understand how Sprint messaging pricing works, but the best comparison to the above numbers seems to be the "Everything Data" plan with 900 minutes, which is $89.99 for 900 minutes, data, and unlimited messaging, 81% of AT&T's cost. It seems to me that the biggest advantage arises on the Sprint published plan rates when one pays for unlimited
everything -- this is only $99.99 on Sprint, and I think the comparable AT&T individual plan is $129.99 (unlimited talk and data) + $20 for unlimited messaging, which makes the Sprint plan 67% of AT&T's cost.