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Lol
Well, I always wondered why the computer would be able to have such a weak economy.

Building maintenance could never reach so high, the most expensive thing can always be the army.
I guess I can only share my strategy, against computer, it does never fails:

Your meta is reaching overpoped cities in a special 'combo' placement, such as having a industrious city with a couple of special resources that provides gold nearby. (i.e.: having a wonder that provides +5 gold in a coostal city with wales or pearls with boats + market + bank + stock exchange + East India company)
Focus on food, some social policies and some wonders, the more population you have in 1 city, the more gold you will get from them;
Don't place cities too near each other, if one city is able to get all the luxurious resources from an area, there is no need to make another one in the same region. No luxury resource = no city.
Latter in the game, if you have enough population in one city with the land properly improved, you will be able to use specialists that will improve your economy further. And latter on you will be able to get social policies from Liberty branch, such as lowering the food cost of specialist, further speeding your city grown and pop cap, and making room for more specialists to come in.

The first building I do is always the monument, and the first policy is the tradition tab. This is very important, otherwise getting social policies would be very slow for your early game and you would be like 30 or more turns behind in that way.

An interesting strategy is to play the game with only 1 city, make it overpopulated, focus wonder building and get the full tradition policy. The population + the policy gold bonus should be enough for you to be able to stand a entire army with a single city, while having it fully built.
Another strategy is to focus the piety policy branch and focusing early game in building up your religion with faith bonuses. Once you have the whole branch, each sacred site you have will give you plenty of gold and culture for any future policies you want to get too. Getting the Economy policy branch can also be helpfull principally if you have many gold specialists that to boost the number of great merchants, but the economy branch is very situational (like the landsneck policy that is good only if you are at war at the right era, or the wagons that is good only if you don't have access to sea from your main trade cities, or the road cost diminished that is good only if you have a great number of roads).

There are many ways to sustain a good economy, and very few ways to lose gold that much; actually to make someone lose that much gold would require certain military strategy.
The real challenge is actually to be a magician of the gold, to be able to fully build an entire city in 1 turn making it reach 10 pop in less then 20 turns. Be able to bribe every single nation of the game to get the votes you would like to see in the congress, save all your currencies and efforts without having any army and buying one when you need, making it pop in front of the enemies eyes out of nowhere while they slowly march towards your "defenseless easy" cities.

If you are having trouble with gold, you are probably not planning properly the place of your cities before building them.
Having a great number of cities can be economically bad actually, it is actually much more military and cientifically purposed. Still it is an interesting way to spend your money once you have an income, indeed, if that is your focus.