well it's not that it drops like candy, m'lady, but rather that snipers hunt atroce all the time whilst LKs don't seem to use them (many other weapons are better in many aspects)... and compared to another of Atroce's 3% drop, namely ulle's cap, those who wish to use the blade (if any) would be very few in number. Thus the cap being appraised at anywhere between 3-6m, i reckon anywhere around 150-200k would be a fair price as i have also bought a few at that price range and the sellers seems to agree with me...
so based on that price range and RO data zone's refining table, i reckon:
+5 = 350k
+6 = 750k
+7 = 1.5M