Money is clientsided..
You can easily change it in some servers using this address: 0xB7CE50 - [dword] Money
However some servers are using the clientsided money only to show you how much money you got, and the real money value is held in a variable on server's side and then saved somewhere when you leave or every few minutes/every time when you buy/sell something.
The money is resetting because all money cheats are using the above address (0xB7CE50 - [dword] Money ) and the server is really using it only to show the graphical money value, and when it changes the server changes it back to the value that is stored in a variable.