HackRF Blue: http://hackrfblue.com/ ~> $200
rad1o (HackRF from spare parts): https://events.ccc.de/2015/07/10/rad1o/ , http://hackaday.com/2015/07/12/cccamp-2015-rad1o-badge/ ~> $100?
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That would only give us half duplex. If we want to simultaneously transmit and receive, I guess we could add an RTL-SDR. ~> $10
Plus antenna(s). $ $ :)