Taylor makes a 5 string guitar that would have a very wide string placement. You could get something custom but it wouldn't be $500. If you are happy with 1 3/4" nut then obviously that is the way to go as everything else would be pretty standard.
Here is a link to Jacob Collier playing a 5 string guitar. It may be a cool option for you but not sure they are production guitars. I build guitars as a hobby and could pretty much build anysize, as most any luthier could as well.
Cheers Peter.
