Boost is $35. Sprint is $85 or whatever with a "free" $150 phone. People have the choice, and they choose to pay an extra $50 / month for 36 months = $1,800 for that phone.
Actually, with proper Sprint service, I believe you get unlimited and uncapped data, instead of throttled at 2.5GBytes/month, and more than that, you can roam onto Verizon's network when Sprint towers aren't in-range.
It's been reported a number of times that Sprint earns more money, per customer, on cheaper prepaid plans like Boost, than they do from contract customers on proper Sprint.
Also, Boost is actually $40 with any smartphone, and even that's only after 18 months of service and on-time payments, so in your first 2-year period, it averages out to $47.50/month, PLUS the cost of the cell phone, which could increase that monthly average by 10-50%.
If you act like most customers, and switch phones and possibly providers every two years, chasing the latest slightly-better deal, you won't come out ahead on a prepaid plan. It's only if you want a cheap phone, and/or are willing to keep it for more than 2 years when it's long obsolete, and/or are willing to stay with your provider long-term, that prepaid really works out for you. I'm happy with it, but I'm not a gadget junkie like many people, so YMMV.