ignoring the cost per mile, I base it on what the ticket would cost. If I could fly from New York to London for $300 then I'll use Mondy, not miles to get there. If it's considerably higher then that, I'll use miles.

I look at the dollar amount and think what fraction of a free trip I'm getting with the miles and bonus miles, so if $300 or so is getting me half a free trip then I'll pay for it. If, however, I'm flying to Denver and that cost is $700 or $900, then it's miles to the rescue.