Why is SDR so expensive?

[der_springer]

I would love to be able to purchase some of the expensive equipment but I just wanted to share my favorite accomplishment with the $20 USB Dongle, a four year old laptop, and free software. I can decode and listen to the local police who are on a P25 system. Record it at the same time even.
Two years ago I had to buy a $600 scanner to be able to do that.
Even without owning any equipment, SDR is interesting as there are numerous radios online that you can have full control over.

[rbm]

You can get a lot of return for the little bit of money you spend on a USB dongle.
I have several of them.

You can use them as a cheap and quick spectrum analyzer, you can even use them to 'characterize' an antenna and/or a pre-amplifier.

I can look at around 2 mhz of spectrum at a time, run the waterfall at a very slow speed, and periodically look at the signals that have popped up.
Using SDRSharp set to 'Snap to Grid' I can see exactly what frequencies have been active.

I've found them to be surprisingly sensitive for what they are.
Here's a short video of one of mine tuned to Milsat pirate activity at 255.550 MHz.
Milsat Satcom Satellite SDRSharp with Eztv668 SDR dongle e4000 chip - YouTube

If you watch that video in HD in full screen mode, you can see that there's also audio activity at 255.450 MHz.
Rich