I just do a lot of things to not need so much meseta in the first place.
Run buster quests - Good exp, helps with ivlida and supernatural 13* weapon collection sheet. Can dismantle 13* weapons for the materials needed to turn vals into atlas's and turn one weapon type into another at zigs. Buster's often drop 10* weapons which are immediately swapped out for ex-cubes. Any unwanted 10 or 11 star units are turned into photon sphered.
Run all Urgent Quests you can - Okay exp depending on the urgent, helps with 13* val collection folder. Can freely get all six copies of your val weapon needed to get it to plus 35, depending on the urgent, drops plenty of 10* weapons for ex cubes, the occasional 14* weapon, and some 10 and 11* units that are again sacrificed for photon spheres.
Do you dailies! - Run once a day on any of your characters to open the boxes - can get unique weapon badges, random items and Bonus Keys!
Run your bonus keys! - The astracite and phantasmilite items you get at the end of the bonus key missions can be exchanged for photon boosters!
Run advanced quests - You should have some advanced capsules laying around. Run a few and you'll get 20-30 photon boosters!
+Ex-cubes - Useful for upgrading to 15*. If you wanna get a couple weapons up that high you'll need about 1200 for three weapons.
+Photon Spheres - Useful for upgrades. When I started I barely had any at all so I dreaded getting a weapon to 10/20/30 cause it eat into my pittance of them. Now I generally have about 50 at any given time!
+Photon Boosters - exchange at the photon booster shop 2nd floor of the shopping area. Buy Exp Weapon Mini these give 400 weapon exp towards upgrades and using these only costs 2k meseta each.
- Using these methods will get you +35 weapons in much less time and for much much less money. So you're saving millions on just upgrades alone. This will let you use your hard earned meseta in other ways. ( I buy way too many outfits but my upgrades and I have several now all cost me less than a mil each)
That said I also have material storage set up using star gems which you should have several of from titles and log in events. I have my material storage set up so most of my mats that drop go straight there. Then I accept the yerks quests in the cafe on all of my character and usually have the item already in storage. Accept and turn in right away fulfilling the "Make more Meseta" weekly mission and bam. 6mil a week from doing barely anything. Which doesn't account for all the meseta I make via triboosts, advanced quests, UQ's, busters, and just running my dailies.
If you can curb your spending on outfits unlike me then you'll have millions of meseta just laying about XD!