If you wanna go for the best monster materials... i've hung out in the Archylte Steppe in the Clearwater Marshes area. I've farmed a ton of different materials from Potent Crystals to a wide array of different essences. I didn't have a real need for mechanical materials, but Academia 500 AF as well as both Augusta Towers drop some nice mechanical materials.
As for the gil issue... ever try fighting the Ochu and Immortal in the Archylte Steppe area??? If you have a durable collector's catalog equipped, these guys will drop some rare stuff that you can sell for some decent gil. But again, I can't think of any reason you need the gil when most of the things that you need are enemy drops.