Just to clarify: the more resources you acquire which are needed for the corporation, the higher its cost, not the opposite.
Okay. To start with, each city to which you spread a corporation contributes 5 commerce per turn (at least I think it's commerce rather than gold) back to the "head office" city--the city where you founded the corporation. So first off, you should ensure you're multiplying that income to help offset the cost of the corporation operating in each city. Make sure the head office city contains a market, grocer, and a bank, and you should also build Wall Street there too.
Second, courthouses reduce not only city maintenance, but also corporation maintenance. Don't spread the corporation to one of your own cities unless it has a courthouse. If you haven't built the Forbidden Palace, build it. Remember that maintenance costs for corporations are cheapest when running the Free Market economic civic.
Next: the chief benefit of spreading corporations to your own cities is not income, but their effects. Each corporation will contribute hammers, culture, research, food, etc. to any city housing it. Before founding the corporation, ask yourself if your cities would truly benefit from the corporation's effects. I myself find that by the time corporations come around, the best two are Mining Inc. (for all the extra production) and Sid's Sushi Co. (for the food and, to some extent, the culture). Creative Constructions and Cereal Mills are my 2nd choices, respectively. Additional production is worth it; hammers are the key to winning this game. Food is also terrific, as it allows you to grow your cities larger, work more tiles and/or run more specialists. You may be able to offset the cost of having Sid's Sushi in a city just by running several merchant specialists. Culture can be handy for cities that are facing cultural border conflicts over nearby tiles.
Finally, one of the best ways to get corporations to pay for themselves is to spread them to other civs. I myself don't like spreading the production benefits of Mining Inc. to a rival, but Sid's Sushi is quite another matter. Spread the corporation to enough foreign cities and you may find yourself able to cripple your rival because they incur the maintenance costs without the offsetting benefit of the head office. Just spread Sid's Sushi to cities that are not next to your own to avoid having that culture boost come back and bite you.