using $5 Monoprice coax cable now, what improvements would upgrading bring?

Yup, you can do better.
I would suggest Audioquest. They have many lines, but even their less expensive lines sound above their price point compared to other manufacturers.
You can call them directly or ask John Rutan at Audioconnection- he know the cables and equipment and can give you great info without any hard sell. 
Bob

I wonder if I use the correct terminology. When I say coax I mean digital coax from the back of my Oppo into the DAC on my Halo.

thanks to all who responded

I should have just asked for a recommendation on a good digital coax cable that does not cost an arm and a leg.

:)

Ah.  Ok.  Here ya go...
https://www.amazon.com/dp/B01N6XL1XW/ref=twister_B01MUH69P1?_encoding=UTF8&psc=1

40 bucks and a very solid, neutral, and natural sounding digital cable with balanced connectors.  If you don't have balanced digital connections on your Oppo and/or Halo you'll need to get some BNC to RCA adapters.  Do not buy a true 75-Ohm digital cable without balanced connectors.  For $40 you can't go wrong with this. 

thanks soix

I am slowly learning...