An SDR is not some idealistic unobtanium. You can now buy SDRs at retail. In other words, this is already on eBay. Below is a list of a few products I find notable. I'm going to count down as a Top 5 just so you are tempted to scroll. If you need to get canonical rtl-sdr.com keeps a hefty list here and RadioReference.com has one here. Prices range from $10 to over $4,000!
5. BitsharkExpress RX - $4,300 [LINK]
This appears to be a good product but I've down-ranked it because the price is astronomical. It supports a tuning range between 300 MHz and 4 GHz and acutlaly uses dual A-to-D converters in a elegant design. Also it is one of the few units which mounts directly on your PC bus as a PCIe card. Thsi eliminates the USB cable many units rely on.
4. HPSDR Hermes Transceiver Card - $895 [LINK]
This open source project came pricey, but also capable of full duplex. It could receive from 10 kHz – 55 MHz but it's A-to-D converter was only 12 bit. Most other systems are already up to 14 or 16 bit.
3. AirSpy - $200.00 [LINK]
Airspy is an advanced open-source SDR that can receive from 24 MHz to 1.8 GHz. That's not everything but that's over 1700 MHz of fun.
2. HackRF Blue - $200.00 [LINK]
This is the low-cost version of the Hack RF. Great Scott were able to achieve a lower cost by better component sourcing. Supposedly the spec its the same but I doubt that a 30% reduction in cost has zero degradation on performance. But I'd love to have the opportunity to test that. See below
1. HackRF One Kit - $330.00 [LINK]
I really want one of these. HackRF is capable of transmitting and receiving of radio signals from 1 MHz to 6 GHz. It's software is Opensource and runs on a on a Debian/Ubuntu distro. "
sudo apt-get install"hackrf"anyone? But seriously this wins best in class. Running in an OS I'm literate in is just gravy. More here.