A couple of days ago I wrote down how much money I made farming all 3 regions in a day, good thing I did because now we can do this.
Before Total: 272,221
After Total: 332,242
Things to consider:
1) I tried to farm amulet for 2 hours but found nothing so I gave up and purchased 2 amulet to use for these runs (I already had one but I didn't use it)
2) Kanto and Hoenn can be done with 1 amulet if you 1HKO or 2HKO between all of your battles, you cannot faint once to complete the 2 regions with 1 amulet
3) I purchased the two amulet off GTL for 28,000 each so 56,000 for 2 runs at this time.
4) You make 60,021 more cash doing all 3 region gym rebattles IF YOU FARM your own coin amulets, if you don't it's barely noticeable of an increase (if you are able to clear / farm the gyms fairly quickly with just 2 amulets).
UPDATE: USING NEW METHOD FOR GETTING AMULETS YOU CAN EASILY GET 5 AMULETS WITHIN 30 MINUTES OR LESS.
Good update for gym farmers :)