I'd say $25. The main value comes from someone that really, really wants it and is willing to pay more than it's actually worth.
I can't imagine one being worth $20,000! Got a link?

Machines that fetch $20.000 are new industrial machines. The very rare collector's item in pristine condition have dollar value, but there are just too many old sewing machines about and way to few buyers for the value to be very high. In the world of sewing machines, only those made prior to 1900 are "old".

On the other hand, these old machines have their advantage over newer ones. They are solidly built and with a bit of time and attention you get them to sew again, and they still do a very good job. The quality is often outstanding compared to new low priced machines, and even if they can't do zigzag or anything other than straight stitches they make a perferct stitch and handle most fabrics with out fuzz. That's why they are still valued as working machines. History and age add to their charm.

I can't help much with dating, it would take a bit more research. Does it have reverse stitching? When you get closer to 1940 most models seems to be able to do reverse stitching, but not all. In the 50s built in zigzag became more common, but it has existed at least since the 1930s on a few domestic models like Pfaff 130. For comparison Singer made a updated version of model 99 (straight stitching only) until the late 60s.

I think the model is "National Rotary C", which looks pretty much identical to National R40. The crincle lacquer was popular for a while, Singer had a few models in black among other makers. If you are interested you probably can find a user's manual on ebay or perhaps ask for a copy from someone. There are youtube videos that might be helpful. If you are going to try it, oil it well every where, just not the motor.

I have that exact model with same cabinet, mine has a book. Model r 32