Short version
Currently, AltGr+- produces ¥. I wish for it to produce an en-dash. In /usr/share/X11/xkb/symbols/us, I replaced all occurances of "yen" with "endash" and restarted, but this made no difference.
For the record:
$ setxkbmap -query
rules: evdev
model: pc104
layout: us
variant: altgr-intl
options: lv3:ralt_switch_multikey,numpad:pc
Longer version
I intially started by looking in /usr/share/X11/xkb/symbols/us, which contains a section marked xkb_symbols "altgr-intl" {. This section contains include "us(intl)" and include "level3(ralt_switch)", and no mention of AE11 (which is the keycode for -).
The section marked xkb_symbols "intl" { contains key <AE11> { [ minus, underscore, yen, dead_belowdot ] };, and so I modified the yen here in the AltGr position to endash. I restarted and this made no difference, so I did the brute force method as above (then restarted).
More troubleshooting
If I set setxkbmap -layout us -variant mac, AltGr+- produces an en-dash as expected (and as defined in the xkb_symbols "mac" { section).
I also tried swapping q and w in the xkb_symbols "basic" { and xkb_symbols "intl" { sections, but this change did not do anything after restarting. I also tried setxkbmap -layout us to select the basic layout, but this also had no effect.