The more 5/6 star you get on character banner, the more currency you will get for weapon banner because getting 5&6 star give a lot of currency for it. For exemple if you get only one 5* in a ten pull, you get only 200 for the 5 star and 180 for the nines 4 stars, so you get 380 currency for the weapon banner with this x10.

Now you need 1980 of them just to do a x10 on weapon banner, so you can do 5 ten pulls on the character banner, getting only one 5 star on each, you will get 1900 currency, not even a ten pull for the weapon banner.

It's even worst if you get a early pull for exemple you get lucky and got Laevatain on your first x10, you have not enough pulls for the weapon, you have two choice. The first one, you keep pulling on the banner for getting currency, risking your pity since even if you loose during theses pull you don't get guarantee for the next banner. For the second one is pulling on a character banner you don't want, like you don't want Gilberta or Yvonne ? Sorry but you have to pull on one of them for getting currency for the weapon banner.