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?
Duelund 16ga speaker wire