I just noticed that Elektor has cut it's prices in a sale! The Elektor SDR broad band receiver can now be bought for €30!!! This will tune from 100kHz to over 50MHz using an Si5351 DDS. programmed from an Arduino.
Put this together with an Arduino UNO at around €9, a small OLED display and a rotary encoder for around another €9, and you can build a very decent SDR for less then €50!!!
Check it out.
I built one, which I use with a low cost 96kHz ADC and HDSDR running on either on my MacBook or a PC.
I have put the software on this blog, look for my latest download of my sketches.