Okay, one way to make some more money is to play cards at the casino. Plus, sometimes guests and friends will call you saying something about the casino's prizes being doubled; when that happens, go back to the casino and you will get twice the amount of money from playing cards.
Another way to get money, is to take the metal detector and go to the desert and find many different expensive items such as spaceship parts, copper, silver, and gold bars, and mummified aliens. All of these can be sold @ the store, but the bars can be processed into money in the vault and you can take the mummified aliens and do an autopsy, selling the organs for $$.