An SS car in total was cheaper to buy than a DS car but at a heavier weight.
That does not appear to be the case with the USRA cars; Ike posted a letter from the Southern files a while back that quoted prices for each: $2919 for the DS car vs. $3050 for the SS car, a difference of about 4%.
Your comments about common frames across multiple car designs got me to thinking; the only other use of the USRA DS boxcar underframe I can think of is the never ordered USRA reefer. In fact the drawing of the underframe presented in the 1919 CBD is of the reefer frame. I'm not particularly familiar with steel underframes for wood reefers; perhaps the design where the wooden carbody posts lap partway down outside the side sills is more common on reefers?
