Originally Posted by Red_Rob

My personal guidelines:

<$.01 --> Never use miles
$.01-$.015 --> Very rarely use (example high likelyhood of plans changing)
$.015-$.02 --> Consider if flexibility needed, etc
$.02-$.025 --> Maybe (depends on total cost, if I need the PQM, etc)
>$.025 --> Use miles

A few key notes:
1) As a 1K/GS trips on miles can be changed as often as you want
2) As a GS I can upgrade even international on award tickets (no W minimum)
3) I consider the cheapest reasonable alternative, not the "Ticket price". For example if I can fly to China on a W fare and use a SWU for $1,800 and a Z fare costs $5,000 I could either book in I for 120,000 miles (+ opportunity cost of 30,000 RDM)... So $1,800 vs 150,000 miles is 1.2 cents. Alternatively I could (and have) booked in X and applied SWU, so it cost 60,000 miles (+ 30,000 opportunity cost) or 2 cents per mile. I did not use 120,000 miles for $5,000 in my valuation (which would have been 4.2 cents).
4) I prefer to use my miles to fly premium cabin on other *A carriers. Its better than UA, and I wouldn't have gotten bonus miles on them, or revenue to count towards GS.
5) The direct answer is $.015 per mile. Much less than that, and I would buy tons of miles, much more than that and I would redeem (sell). For example if there was ever an offer that let me buy miles at $.0125 (without flying) I would spend a few thousand dollars buying miles.