In FFX, Don Tonberry is for free sphere levels.
Equip your characters with Auto-Phoenix armor, and weapons with Overdrive-->AP, and set your Overdrive charge mode to Comrade. Every time Don T uncorks his nasty special attack against a character, the other two send that big bolt of damage to their overdrive gage, and from there to their shpere levels. Once you get low on Phoenix down, flee. Side note: Big Don T's special attack seems to be based on how many things you've killed, the warriors in my group, like Wakka, routinely get pummelled to the tune of 99,999, while the wimps like Yuna get 'only' 12,000 damage. I suggest that anyone you plan on having train this way be put in your fighting group to increase the amount of damage that Big Don T will do to them when they face him. (I know, sounds crazy, but that's the way it works best.)
If you need Mega-Phoenix to make Auto-Phoenix armor, bribe the ghost-creature in the Sunken cavern where you get that useless Aeon that you have to pay to fight, and it will give you 30 or 40. It costs about 300000gil.
Getting Overdrive-->AP is harder. Mech Leaders will give up Door to Tomorrow if bribed(~50,000 I think will do it), and there is an arena monster that you can steal them from I think. Use these to customize a weapon with the Overdrive-->AP ability. There is an arena monster that drops weapons that have the ability built-in, but I think it might be too tough for people who don't already have the Sphere Levels gained from getting pummelled by Don T already, so it's kind of moot. It's probably a LOT cheaper to steal them from the appropriate monster, even with the high price of fights, than bribing those stupid Mech Leader's, although if you're looking for Al Bhed potions, that's an easy way to kill two birds with one stone, since you can steal from his guards to kill them. (Stealing from a mech kills it.) I think once you unlock a second monster, the arena master gives you 99 Door to Tomorrows anyways, so that will save you some time and effort. It might be a particular area that you have to collect all the monsters from instead, I forget.
If you can put Overdrive-->AP on a weapon with Double or Triple Overdrive, and/or Double/Triple AP, even better. Even SOS Overdrive helps, since if you have 9999hp, after Don T's first blast, you'll come back with less than half life, and get double overdrive. Using this trick, I can walk in to a fight with Don T, spending 8,000gil and about 50 phoenix downs(another 5000gil), and walk out with at least 3 pcs with 99 spere levels (maximum you can have at one time). 297 sphere levels for 13,000gil. Now THAT's good business!
If you need money, one of the best sources is the boxes in Omega dungeon. If you see a lizard with a box, steal from the box, and it might be a mimic instead. If you have a character with a double money weapon in the party when the mimic is defeated, you will get 100000gil for the mimic, and some chump change for the lizard. Just don't kill the lizard before you open the chest, and watch out for its petrification attack.
Another good source of money, once you are a god with Break Damage Limit weapons (hopefully Ultimate Weapons, but some of them are so hard to unlock as to be worthless (Kimahri and the kid), is killing arena monsters and selling the awesome armor and weapons they drop. Some of them will drop massively expensive armor or weps, just fight them alot, and sell the goodies. The floating eye (One-Eye?) drops really good ****, but Beserk and Confuse-Proof your dudes, or you'll never beat him. I think he also drops Magic Defense Spheres, which will turn empty nodes into Magic Defense nodes.
Personally, I had to save up my Dark Matter and make 'ultimate weapons' for Kimahri and the main character. Kimahri's has BDL, 1MP Cost, Magic Counter, and Evade/Counter. The other has BDL, First Strike, Evade/Counter, and I forget what else.
I hate blitzball. I hate it a LOT.