miketsp:I only gave the example of the echosounder to show what can be achieved. If they can sell it for that price the real cost is much lower.
I wouldn't use it as a basis for such a project.
A quick search on the internet for piezo ultrasonic transducers turned up a variety of products from very cheap to very expensive according to the type of encapsulation, precision and frequency range.
I would certainly not expect to spend much over 100USD on electronics. The real trick with such a project is to get the right balance between analog and digital processing.
Fair enough. Are you a hobbyist or an engineer?