Playing with it in the US and using ITA Matrix to return Y Fares and knowing that some of these are shortest routes in the US with regularly scheduled service:

AA: DFW-ACT (Waco, TX), $1017, 89 miles -> $11.43/mile

UA: SFO-STS (Santa Rosa, CA): $649, 66 miles -> $9.83/mile

AA & UA: ORD-MKE: $649, 67 miles -> ~$9.67/mile

UA: DEN-COS (Colorado Springs): $649, 73 miles -> $8.89/mile

DL: DTW-LAN (Lansing, MI), $599, 74 miles -> ~$8.09/mile

I ignored the "250 miles" rule because in the US, two of our largest and most important cities, NYC (LGA, EWR, and JFK) and DC (DCA) are about 200 miles apart yet have a high number of O&D traffic.

On Monday, 18 July:
LGA-DCA (214 miles) on JetBlue: $857 or ~$4.00/mile.
EWR-DCA (199 miles) on United: $659 or ~$3.31/mile
JFK-DCA (213 miles) on JetBlue: $979 or ~$4.60/mile

Similar for NYC-BOS, another city pair under 200 miles in the US with high O&D traffic:
JFK-BOS (187 miles) on JetBlue: $1245 or ~$6.77/mile
LGA-BOS (184 miles) on JetBlue: $1324 or ~$7.20/mile
EWR-BOS (200 miles) on JetBlue: $891 or ~$4.46/mile


Looking at our neighbors to the north in Canada for a couple shorter routes with air service:
YOW-YUL (Ottawa to Montreal) (94 miles): $1113 Canadian Dollars = $854 USD = $9.09/miles
YY-J-YVR (Victoria to Vancouver) (39 miles): $630 Canadian Dollars = $483 USD = $12.38/mile