I personally believe that exchanging (like this) isn't quite the right way to obtain a rare item like this (see bottom).
The issue with those kind of items is just the pure luck that is required. Some people will drop it in 10 runs, some people won't drop it in 100.
With this you could of additive rate added per run (that you have a higher chance with each run you do). Though, that would require time to implement and since we are working on big updates at the moment, it's not a priority at the moment.
Else you could just think about the concept of "rare" items in the first place. Farming enough until you have enough mats for the weapon could be a thing, just needs to be made properly (and the way Drakos and Van Cliff work it's not that good :|)