Can I ask where you found out the name to be Fire at Will for GMS? Thanks!
There is already a feature for this for your world. If you look up an item on the AH and it is there, you can select it and press the "Search(?) Market price" button in the bottom right to see price trends over a limited time (at least 6 months).
Grinding is at least more helpful for your character though. I'm trying out a lot of characters atm and it's much preferable to be able to level up while grinding for ARK coins. Unless I'm mistaken, Fishing isn't granting me any exp. That's why I prefer the field one. I'm much more into grinding than playing games that aren't helping advance my level. Plus I find the fishing game to be tedious and boring, frankly.