A million frequent flier miles via a major alliance airline is a very, very sweet prize. That's enough for a person to fly themselves and their spouse to basically anywhere on the planet in business class five times, round trip.
A good rule of thumb for the floor value of miles is a $0.01 per mile (i.e. a penny). From a value perspective, you usually get a better exchange for business and first. Of course, you may or may not normally pay for those upgraded seats if it were your own cash so imputing the actual value is difficult in that case.
Some airlines are moving to systems that tie reward miles more explicitly to both cost of tickets paid for and value of tickets being redeemed for but the above scenario is how it's generally been done to date.