I don't buy in the volumes Sprint does, but they're not going to get a ton of discounts if there are build costs rolled in. On the contrary, it is easier to absorb the build of a random circuit here and there vs. tens of thousands of them. Your random GigE of backhaul will likely go for $1500 - $4k/month, depending on build requirements. A second GigE on the same route is going to be significantly less. A 10GigE is going to be $3k - $8k. I've had 10GigEs com in under that. If they were smart, they'd go from GigE to 10GigE and use some microwave backhaul to aggregate sites.
Verizon has been moving to raw dark fiber, allowing them to light it themselves, be it 10GigE, 40GigE, Nx10GigE, etc. Obviously those are all that the very high end of what is needed for cell backhaul outside of a venue, but Verizon will have more control over their network and their pricing by going dark.
Dark in a 20 year IRU format will range from $800/strand mile to $10k+/strand mile, all depending on routes. Difficulty and competition play the big parts here. I thought I saw it was a $145M deal, but I can't find that now.