Tl;dr: Take the price you’d realistically want to pay for a particular flight, then divide that amount by the number of miles required to redeem that flight. It should give you an indication of how much miles are worth to you.
This is probably a rather contentious topic, and also one that has been explored by quite a few people online. I feel the need to chip in as my valuation of miles is quite different from what I’ve read so far, and also the fact that knowing the value of miles is very important to guide what cards you put your spending on.
It seems to me that cashback lovers tend to ignore that miles have value, preferring cashback even when it’s a poor rate, while chasers typically overvalue them at almost 2 cents per mile. My personal valuation of miles, specifically the KrisFlyer variant, is