Let me break this down for you people since I have one of these weapons.
There are hardly any of these weapon types made. The only common Double Attack weapon is the Katana anyway, since there haven't been any good high damage choices except for trial weapons for the weapon type. Dagger has better choices, Axe does, Sword does, Club, etc etc. Not a lot of people are going to want to buy these.
50k is a pipe dream. (10mil to get +2dmg)
10k might probably be too. These are just going to end up getting tossed nonstop for sure.
Double Attack dagger is the best mainhand of the "easy daggers", which basically means its the third best mainhand of all daggers, sitting just below Curoscanti and Twashtar.
Second, doing the silver mirror trial bumps up the dagger from rank 4 to rank 5, making it a very key amount of dmg to have on a dagger.