Was probably reported, but can't find it.

Cost to complete should be: 4125 - quarterstaff(900) = 3225.

In this case however it removes twice the price of quarterstaff, one for each of the Oblivion Staffs, which might be misleading to players.

Same story with Pipe of Insight if You have 1 Ring of Regen which is needed both in Hood and Headdress.