I think the rule of thumb used to be higher. Pre 9/11, the most expensive common routes in the US domestic tickets were around $400 for coast to coast, but with the increase in discount fares cutting this substantially, IMHO 25,000 miles is worth $250 or less. This is based on me trying to trade RTFC (free round trip vouchers) for $200 vouchers on the same airline to no avail. However as some have pointed out, the value in miles to money is not linear. If you spend in excess of 80,000 miles for an award, usually flying comfortably over a large body of water, the value of the ticket is worth many, many times 3 US domestic awards.