FarmVille cash is different to coins. It is signified by the notes symbol next to the coins at the top of the screen.
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Every time you go up a level you are given one more unit of FV-cash. You can also buy FV-cash online.
Another way to get more FV-cash is to fill in surveys on this facebook page facebook.com/onthefarm/ This can bring in a lot if you are quick (and maybe unscrupulous) in filling in forms.
What can you use these riches for? Well, there are some nice garden gnomes that you can buy for your farmyard plus a flamingo or two. Then are really useful things like seeds etc...