For a pure MR:

<4cpm and I will think about it if it suits my available dates (usually Saturday same-day turn) and i like the routing.

<3cpm I am definitely interested and will be a lot less picky.

<2cpm and I will bend over backwards to get in on the deal, mostly for personal satisfaction.

Equipment flown, likelihood of upgrades, lounge access, transit airport all come into play too.

As soon as it's a VR, all bets are off and I will happily pay 5cpm if it gets me to a place I want to go.

And I concur with ddrost1 that sometimes all bets are off. One year I miscalculated and needed 1000 miles for gold status. Paid 12cpm in late Dec for a quick turn but IMHO it was $120 well spent.