This restores GPs yield of #kularzunevelafrill

Ledmitz nerfed bone arrow price in one of previous commits:

NPC -> player vending price: 50 GP -> 8 GP (nerf factor of 6.25)

Player -> NPC selling price: 20GP -> 4 GP (nerf factor of 5)

This commit re-scales yield of spell to restore GP output, roughly same as its been before nerf.

Rescale factor choosen 5.333 which fits between of 5 and 6.25 nicely and convenient as it allows just minimal /32 -> /6 divisor change.

As far as I experimented, yield remains roughly same, about 740 vs 756 GP on my setup. Slightly imperfect due to integer rounding, if this is a big deal I can use more complicated formulae.