Rumor has it someone leaked GLs bid amount (to Richie Bros) for the contract and Richie was able to win.
It's been hinted at that there was some "tomfoolery" on both sides including GL suing after the contract awarding, but the thing that sold Uncle Sam most on the original two contracts (basically rolling stock & non-rolling stock to keep it simple) was the % of return to Uncle Sam. For the rolling stock, GP purchased the items at up to 6% of acquired value and Uncle Sam got 75% of the bid auction price. This did not include the GP fees, SF-97 costs, etc which went straight to GP. You can imagine GP playing with the numbers and they can really get some margin in there for themselves.
It gets interesting when items were wiped off the books and DTID tags show value as $0001.00 dollars or etc. You can see where some items made big money and others made GP eat their own hat like the piles of wall material that went up for auction. The best thing for them was consistent manageable product with relatively low acquisition cost and no civilian market supply to compete with. They can legally do anything they want with the items turned over to them including not sell them (but they eventually will if the price is right).
If you've noticed some auction items were changed from "Sold" to "Buy it now" pricing, those are when the buyer filed a claim against the item and instead of GP eating the difference they altered the reported price back to Uncle Sam so he received less as the item was technically resold at a lower value to compensate for the issue. Again, they can play with the reported numbers.
Now, all this is old info and only applied to the original contracts. When GP won the new contract, yes both contracts were combined into a single contract, they were able to get better terms for themselves. This was in part to no other bidder on the contract due to terms (like having $200m in liquid capital), but also Uncle Sam has been reportedly satisfied with how things have been handled financially and physically.