part of the reason thermal cameras are cheap and ultrasounds are not is that thermal cameras have consumer uses. One can use thermal cameras for hunting or determining home energy efficiency, an ultrasound can pretty much only be used for medicine. Although, consumer sonar for fishing(which operates at lower frequencies) is still fairly expensive[0].

Second, your estimate on the price of PZT elements is not necessarily realistic as the transducer is rather large(22 mm in diameter). A 10X10 array would be impractically large. One can buy PCB mount piezo elements for motors for cents per element in bulk[1]. It might even be possible to modify one of those expensive components into an array as was done in this homemade STM[2]. Although in both cases there are bound to be issues with impedance matching and potentially changes in resonant frequency.

[0] http://www.westmarine.com/sonar [1] http://pcbmotor.com/applications-and-solutions/smd-piezo-com... [2] http://dberard.com/home-built-stm/