I know, if you can't pay the fine don't do the crime, but realistically income is not a constant and is much too temporal to be useful here. A decent income year last year, and little income this year, means you might have no hope of paying the fine.

Why not a fraction of the current value of gross assets at the time of investigation? This a much better reflection of someone's financial standing, is a greater deterrent, ensures that the fine is actually payable, and deals with the issue of the rich not having much in the way of income.