There are several ways you can make money:
(1) Build or upgrade new residential buildings. Be careful to include new residential buildings within your existing service zones so that you don't have to worry about needing too many new services.
(2) Collect taxes on the residential buildings that you have. This is much slower than other ways to make money, but over time, if your sims are happy, will provide a good, steady payout.
(3) Trade items on the global exchange. Really good rates go for items that are needed to expand city storage (ladders, security cameras, padlock), to expand map areas (dozer exhaust, plow, gear), and for doing disaster events (gloves, buttons, batteries). However, you can sell ANYTHING that you manufacture or create in a factory on the global exchange.
(4) Wait for simoleon bubbles to pop up and complete the trade that's asked for. Many times it will be for manufactured goods, but sometimes they ask for expansion items. If you don't have the items right now, you can manufacture the goods and go back to the bubble later and sell the items when you've completed the manufacturing.
(5) Do cargo shipments on either the boat or the airport. By fulfilling those items not only do you receive simoleons but also keys and other items to purchase specialized services that are unlocked later on in the game.
Also, in general, it pays off in the long run to purchase the most expensive police stations, hospitals and fire stations to encompass the largest area. Also, it makes sense to invest in better sewage, water and power options. Then you don't have to worry as much about making sure you're covered.
Best Answer
Those prices are set by the seller of the item. When you go to the exchange you are given the option to set the price that you're willing to accept for the item.
I've also found that if you set the price too high and no one buys it, eventually the system will buy your item and your selling spot is freed up for you to sell more items.