Now the definition of "complicated" can be subject to debate, but you could be surprised if you compare the production prices of simple/high-end cars and simple/high-end ultrasound systems.
Really high volumes help a lot for all components. There are more and more components in ultrasound system that can be reused OTS from other markets, but even then, those larger markets are not necessarily larger than the cars market!
And it is not too dangerous to build one yourself for non-medical use, although it might be a little harder than you seem to think. But if you are reasonably skilled, I believe it is doable to design a toy system reasonably cheaply (but simply, do not expect to be able to use a raspberry pi GPIO and obtain something non-shitty). Go on if you want!