Roulettes will get you that. Join roulettes as jobs that are in need and lv80+. The clusters can be traded in for X and IX. Usually at least one of the main three types will hover around 10-15k per piece of X...

Expert and lv90 roulette dungeons also drop a ton of IX and X.

If you don't mind following a train...each day there are 10+ A trains going per data center and on Tuesdays it is double that. Each full 16 mark train will net you on average 10 X and 10 IX clusters and depending on the conductor, it can finish within 20-35 mins. Also there will be a lot of tomestones and nuts that can be traded for even more materia. This alone means ~30 mins you make 150K to 200K. This is seriously the fastest way for a DPS noncraft/gathering player to make Gil...on Tuesdays, if you are serious about it and do it the whole day. You can potentially hit 20 trains sometimes and that is 3-4 million Gil (I have never done that, max I did was 6 trains).

I'd think 3-4 million Gil can outfit 3 different job types on 610 gear...pretty much all you will need unless you want to outfit every job.