Eh, I didn't take text messages into consideration, so ok, the Pre is a bit cheaper per month.
I was going off the base advertised pricing (and I always thought the base voice plan was $29, not $39 for iPhone). And face it, many consumers don't pay attention to details or fine print. They'll see the $199* for the Pre and assume they get it for $199 out the door, which they won't unless they go to Best Buy. Those are the same people that will see the voice+data plan price between providers and not pay attention to who includes SMS and who doesn't (unless they have kids that send a gazillion text messages a month). Just like the tons of customers Sprint is counting on not sending in the mail-in rebate.
Another thing to consider is that AT&T and Verizon give discounts to just about everyone under the sun. Sprint and T-Mobile don't give them out as much. Most people in government, education, large companies, etc get AT&T or Verizon discounts. A lot of those places do not give Sprint discounts. I can, in all honestly, say that I don't personally know a single AT&T customer who pays full price for their monthly bill. EVERYONE I know has some sort of work or school discount with them.