It only takes a single person figuring out how your save system works and telling the entire world about it to ruin any client-side protection you try to use.
The only way to prevent the player from manipulating their save data is to perform all important actions (even gameplay) on a server you control. In other words, the way an MMO does it. The cost of actually doing this scales with the number of players you have, even if the game itself isn't an MMO. It will also piss off the players, because they will have to be online all the time.
It all boils down to: How much effort and money are you willing to spend to stop cheating?