There are several ways to make gold, usually through creating items or playing the games.
Also, not all items require formulas. I remember my old way of making gg was to go around capturing bugs in Towns (the first one, not Towns 2). If you get up to, if I remember correctly, 50 bugs of a certain color, they can be made into a dye. Dye can then be made into tattoos (i'd suggest collecting the dyes a bit, as you can make some really elaborate tats when you have a large quantity and variety)
zOMG offers many coins, but it depends on what you're doing. Play that, and the best advice I can give you is to join crews to fight the area bosses (Papa Saw in particular gives you about 6k per run).
Slots and Cards, the games, are games that require tokens to play. These games are played in order to cash in for tickets, which tickets can be used at a shop called "Prize & Joy". Not many do that anymore, but go play any tokens you might get in the daily chance. You get lucky once in a while, and you can sell those ticket items for a fair price.
Item creation with formulas is somewhat new to Gaia (or to me at least) so I don't have much experience there. But, it's basically just collecting certain items to make whatever item the formula is.
Frequenting forums, and voting in the art gallery can get you some gold, too.
And uh... I'm just babbling on, i'm sure there's other comments going over what I haven't. Best of luck!