I think it's telling how much DL thinks the PMUs are worth.

In lieu of 4 PMUs, you can get 20k miles (worth $200 to most people) or 4 SkyClub day passes (worth $45 each) - putting the value at $45 per PMU.

In lieu of 6 PMUs, you can get 50k miles, which would put the PMUs at about $83 each, or 6 SkyClub day passes, still $45 each.

Judging by the difference between DL M fares and other airline discounted business fares, $50-$100 seems about right. Of course, you have to give DL $2500ish to use a PMU, and you can give them $0 if you just buy a ticket on another airline (who will eventually give you a real system-wide upgrade for your loyalty).