Mic for recording sound demos ?

I believe that you are talking about recording sound coming out from the speakers of your stereo system. I have seen a YouTube video about such recording but from one speaker only with a blue yeti and the result is quite impressive. See  https://www.youtube.com/watch?v=or-cwTKmk_8   For stereo recording, I think that you either have to use 2 microphones, one for each speaker, or if you use the stereo function of the blue yeti, you may have to put the speakers close to the microphone. If the microphone(s) is too far away from the speaker(s), the room acoustics will play a great part in the sound received by the microphone(s). 

Yes I actually did this and was really impressed with how good it sounded! The stereo mode of yeti.

The Yeti is easily good enough to do the job and will outperform the dynamics especially at high frequencies... the built-in ADC pretty good as well. I suggest you check you're getting the most out of the Yeti by checking your gain settings to ensure you've got a healthy input level and the signal is not clipping. Also check the recording parameters - bit depth (ideally 24) and sample rate (at least 44.1kHz).

By the way, the Yeti has three capsules so is in effect three microphones... these get mixed together to produce the different frequency response patterns. If you don't have a whole lot of experience then a single stereo mic like the Yeti will help you avoid the hassle of trying to phase match a spaced pair of mics (look up xy configuration if you're going to use two mics). 

Get in the near field, use stereo mode

I have a Zoom H6, really a bad a bit of kit and way more portable than the PC thing....

have fun

Ya I looked hard at zoom h6, h8 and sound devices mixpre 3 II.
As for stereo mics I considered rode NT4 and for separates Rode M5.

I have an MXL V67Q stereo mike, which I use to record myself on piano, fiddle and guitar. I connect the mike to my Zoom H4 portable recorder. Not that the Zoom’s attached mikes are rotten or anything, but the MXL actually gives me bona-fide tone and imaging. By the same token, though, it still ain’t quite out-and-out pro quality.

A while back I did some research on some lower cost microphones that had really good sound and response.  I ended up getting a matched pair of Rhode M5 mics, which worked really well.  They are well regarded by musicians for general recording.  By coincidence, I also used Focusrite 2i2 for mic interface, since these mics require phantom power.

If you want to step up, the Rhode NT5 are excellent and also include a really nice case.  They are over twice as expensive (but still under your $500 limit).