Dude, yeah sorry I don't speak German. Also your English isn't exactly piece of cake.
My understanding of an SDR is a combination of software with an hardware interface.
This camera you gave us as an example transmits at 2,4GHz using 18MHz bandwidth. Good luck finding a cheap RF interface with those specs.
If by SDR under 10$ you mean RTL dongles remember those usually won't go as high as 2,4 and get much more than 2M bandwidth.
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