See my post possible kit for sale and read below.
To build a sonar is as easy as the answer to how long is a piece of string!
Do you understand how they work in intricate detail as this is the key to designing one.
What skills do you have?
You need to be able to program
You need to be able to design hardware
What facilities do you have?
You need to be able to build hardware
You also need to be able to test hardware.
answer these first!
Next Cost it will not necessarily be cheaper to build your own just interesting. When you first start the results will be very mediocre It will take years to perfect your system.
Finally if you want a professional system you need the right bits. In a camera the quality of the image taken is only as good as the lens. In a sonar it is only as good as the transducer and to buy good quality transducers costs 15000 pounds each for dual frequency units and you need two 1000 pounds each for single frequency units. these go down to 900 pounds and 700 pounds in quantities of 10 or 20 depending upon the company. You can make them but they are hard work, involve elements fitted into an array which costs no trivial amount. and once you have made it if you calculate and get the spacing correct to prove it you need an underwater acoustic test range with minimal turbulence and noise and a calibrated transmit and receive system to map the beam width, azimuth, signal to noise ratio and matching. People then normally produce a polar plot to show the lobes and radiation patterns. A transducer designed wrongly will put its energy into the side lobes and waste power as some case designs prevent this being transmitted through the use of an aperture to focus the beam.
You get more gain out of building and designing a professional system than a cheap one as some cheap sonar on the market are 2 thousand pounds or about 4 thousand dollars. I'm afraid third world companies can out do the western world for manufacture costs. I know I can build/assemble a professional system for little more than the price of the cheapest system on the market.
The best bet IF you want to build one is to buy an old eg and g 272 or klein type fish and modify it for digital with kit like mine which is quite cheap. Alternatively buy two transducers and construct your own fish around hardware like mine.
If you want to construct hardware I can give you the information or sell you the bits at cost plus postage and maybe a bit more depending upon how much effort I have to put into the next generation of software currently on test.
Im not in it to make money yet but It woudl be good to help others out.
Robin