DC's Metro is currently in the midst of adding about 700 new railcars to replace/expand their existing fleet, and the cost is about $1M each. This includes a lot of features (like signaling systems for regular operations with active switches between lines, onboard automated station ahead signage, etc.) that wouldn't be necessary or as complex on a system like WDW's where each line operates independently of one another during passenger service.
As a rule of thumb, steel wheel vehicles (like DC's Metro) tend to be larger and more expensive than rubber tire vehicles (like WDW's monorail). The larger size means bigger requirements for drive systems, operating speed, ventilation, vehicle body support (and in turn, crashworthiness), etc. Metro's cars are 75' long and carry about 125 passengers each (seated and standing); WDW's cars are about 35' long and carry about 60 passengers. Both systems have similar operating schedules, so durability of parts wouldn't be a major factor, as is sometimes the case when comparing Disney attraction costs to its competitors who have shorter and/or seasonal hours
Obviously this isn't a direct comparison (there's no such thing as an off-the-shelf railcar, they're unique to every system), but it seems reasonable to think that $1.4M per car could certainly be in the right ballpark, particularly for a private company that isn't subject to the Buy America clause. If anything, it seems a little on the high side, though the relatively small size of the fleet and relative uniqueness of monorails may contribute to increased unit costs.
