I can't speak for Hubway, but most municipal bike programs I've seen have bikes built like tanks, which must cost more than $50. I assume this is to minimize the maintenance costs that must be inherent in a system where the bike users have nearly no incentive to take care of the bikes. And, of course, you still have maintenance costs even for these tank-bikes.
Hubway bikes have often been described like that, as "tanks." They are 42 lbs each, designed like that on purpose to be stable & steady street bikes for riders of different skill levels. They do, indeed, cost more than $50, but a lot of that is the technology involved with the docking/rental system, moreso than to minimize maintenance. Alta Bicycle Share, the system that operates Hubway for the metro-Boston region, employees a full team of mechanics and field staff to do regular maintenance, including a full service during the winter months, on the entire fleet of 1300+ bikes.
Here in Hamilton we're getting SoBi - their bikes actually contain cellphone and GPS components so they can phone home with their position. That's more maintenance and service costs.
