Graybeard, that isn't the case at all....
The 757 has roughly half the fuel capacity of the weaker KC-X competitor, the 767 NoGo Tanker.
But 2 x 757 burn a lot more fuel than 1 x KC-X, they'd need twice as many crews to pay, feed, water, house and look after as 1 x KC-X, they'd need VERY expensive modifications to fit a boom (if indeed that were possible) and a FRU. The wingspan is very tight for fitting wing AAR pods, that's if the structure could even cope.
Add in basing, training, military avionics, data link etc etc - and your boneyard bargains soon become VERY expensive......