The problem is then the rich will do a simple expected value computation to decide whether to do fraud. A disproportionate and non monetary punishment will be far more effective at deterring them.
Then write the law so that the penalty can never possibly be worth the cost.
Jail is downright inhumane, and the only reason we should tolerate it is the case when the person is violent and there is good reason to believe that in the absence of imprisonment they will be a danger to others. And they should remain in jail only as long as the threat of danger remains credible.