To cover you had to provide USD. Since USD is fungible the client can't know the price you paid for the USD you're using to cover. So the profit or loss is the difference between whatever you had to pay for the USD vs what it cost you to short it.
So an example using easier to read numbers.
BTS peg is at 0.005 BTS/USD. You short $1 and someone takes the opposite side. So you have 200 BTS for the short , 200 BTS collateral and about 200 BTS from the sale oft the minted USD when someone took the other side.
Now weeks later BTS is selling for 0.008 BTS/USD. You buy $1 of bitUSD for 125 BTS and cover. You get your 200 BTS back for your short, your 200 collateral, and the roughly 200 BTS you made from selling the USD. So your profit is about 75 BTS.
So in your case you need to look at what you put up to short the bitUSD and then what you paid for the bitUSD to cover.