You are confusing several different thing under the vague notion of "ultrasound machines". Doppler mode is useful and cheap machines dedicated to that can and do exist. However even a basic machine able to do only a poor B-Mode needs at least vastly more processing power -- and vastly more channels.
Processing power is not a problem these days. Maybe if you feed all channels at ultrasound frequency to ADC and then try to do all processing in software. But you don't have to, it's oftentimes easier to have hardware (FPGA) pseudorandom signal generators and correlators to do the bulk of signal processing for you.
It depends. An FPGA able to do that is a big one, quite expensive. Today, an even more so tomorrow, you can afford to do that kind of computations in a CPU and/or a GPU.
Regardless of who is doing the beamforming, the TX and RX analog parts are intrinsically quite expensive with at least dozen of channels.
