You have to distinguish between price and value. You need to first decide why you want the miles and how much they are worth to you. A couple of (trivial) examples:

Whatever price/mile you manage to find, value/mile is zero if you don't use your miles before they expire.

If you are a few thousand miles short of an elite level, a short mileage run that puts you just over the threshold may be better value than a long (overkill) run even if the price/mile is higher.

If doing a pure run, without the slightest interest in the destination, you need to factor in all other costs as well, hotels etc. Better value IMHO is usually to try to combine your runs with a mini-vacation somewhere you actually want to visit, even if price/mile is a bit higher.

Obviously, it's all highly subjective...