It also depends on how many miles you have. Once I get above 300k miles in the bank, 1 CPM is my threshold to decide if I use cash or miles, for short flights that don't impact my MQM much (have you priced DTW-MSP lately?

).
I love to use miles for J awards internationally (when I can find them!) but I don't use CPM there because I would NEVER pay for a J ticket myself.