For me, the value of the miles/points is equal to the cash price I would be willing to pay for the room/flight (up to the actual amount), less any portion actually paid in cash.

For example, take a room where the nightly rate is $200 cash or I can pay $80 + 10,000 points - if I actually would consider paying $200 for the room, then my points are worth ($200 - $80) / 10,000 or $0.012 each. If the most I'd pay for that room is $150 though, then my points are worth ($150 - $80) / 10,000 or $0.007 each.

Same logic applies to plane tickets where there is no cash portion. Let's say a Business Class ticket currently sells for $7,500 cash or 125,000 miles. I'd only be willing to actually pay $3,000 for that ticket. I value the miles at $3,000 / 125,000 = $0.024 each. This stops me from overstating the value of my miles when the cash price is something I'd never pay.