Restaurant City money is made in a few different ways. The first way is through serving meals to your customers. You get 2 pieces of Restaurant City money for each meal served. To make more money, serve more customers by expanding your restaurant by using a functional restaurant layout and using a few more tricks you can find on this site. A high popularity rating will keep more customers coming in for your employees to serve. Restaurant City money is gained while you are logged in the game, or while you are logged out – however your employees must have more than 0% energy to work.
You can also make more money by cleaning up your restaurant or your neighbors restaurants. Just visit your neighbors restaurants and if you see trash on the floor, click it and you will gain a coin. The same goes for your own establishment. Unfortunately, you cannot gain money from cleaning toilets. Also, you can shake trees for money, or by setting up arcade machines in your restaurant.
There are a few cheats out there that help you make money, but use these at your own caution, because they can sometimes cause more problems than they fix.