Looks like there are a lot of open source SDR related things on the web. But the more powerful SDR hardware is expensive. The USRP is part of the GNU Radio project and it is $700 another radio with this price tag is the QS1R.
Now the question is, would it be possible to connect some of the components found in those devices modular together.
Looks like the LTC2208 is used in a lot of projects like this for ADC, the price for this hardware alone is $70 to $160 depending on the version and pieces. I’m not sure if there is a breakout board for this chip?
Another component found is a FPGA, the XuLA project has FPGAs between $39 (50,000), $79 (200,000)and $200 (1,000,000). VHDL is required to use the FPGA, I did order a $39 version – but I never used FPGAs and VHDL. I found this tutorial I will may use to learn something new.
A micro controller is also found on those board.
A onboard DAC could also be used to listen directly.
Gigabit Ethernet, USB or some other fast method to transfer all the data to the PC. Could SATA be used? Or firewire?