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

using Oppo BDP 83 to Halo integrated with the cheap coax and hoping to get a step up in sound without spending too much.  <$100

suggestions on what to buy?

Going from that interconnect there are a lot of things that could be "stepped up."  What improvements would you like to achieve?  More detail, richer tone, tighter bass, better imaging/soundstaging, etc.?  Considering it's the only interconnect in your system, I'd spend at least a little more than $100 -- it'll be money well spent IMHO.  What are you using for speaker cables?
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. 

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...

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.