It really depends what the cash price is for the equivalent ticket. The way you would figure it out would be once you know which mileage ticket you want, try to find a broker that will sell you miles.
Good point.
According to ITA Matrix, the cheapest round trip would be $970 Canadian if i left from Toronto, or $1058 if from Detroit. One way, is $799 Canadian from Toronto, or $923 from Detroit. I'm checking for Toronto, since it's just a few hours away, and can be worth it for some flights.
For this, i guess i would work with the round trip cost from Detroit, because if i paid cash, the second way costs just $135. I am not sure i would buy both now with miles, because i may not come back then. (Unless it is a deal i should not pass up.) Anyway, let's compare:
RT vs RT: $1058. Subtract the ~$145 that the Air France has anyway, twice, leaving $768. Cash vs 50,000 miles. 913 / 50000 = 0.01826.
OW vs OW: $923. Subtract the ~$145, left with 778. Cash vs 25,000 miles, 778 / 25000 = 0.03112.
Am i doing it right so far?