These have been my tests on this armor piece. Farmed using blm. Didnt have to engage mob, i just macroed in the neck in my stun macro so the armor will be on for that one spell, and thats all i needed to have gilfinder effect on mob. i fought the 3 ant NMs (20min respawn) in Quicksand Caves they drop 6k gil everytime. With this Gilfinder under effect its always been 9000-9300 everytime so for me it was worth it, just throwing this info out for whoever is thinking of buying.
When Gilfinder procs (and you have to engage with weapon to get it to proc all the time) it goes for +70%-120% of what the mob would drop, not +60 gil per mob. This is a great Piece for gil over time if you are farming mobs for gil, I have used this on Tonberry and Shagin nm and it makes a normal 2k drop to 3900~.
Again, this is not a +50% deal (my Experience is it is closer to a +95% thing over time), It is a great piece for the committed non thf farmer who is interested in farming mobs that drop gil. I worked off the 200k it cost on mine in 2 days of farming NM.