The penalty for serious abuse of the system's trust is frequently greater than whatever consequence the offender had been trying to avoid. Examples (all, coincidently, drawn from NSW) include Marcus Einfeld's speeding tickets, a lawyer struck off the Roll for telling lies to get his clients' cases moved to another venue, and the Bankstown probation officer who gave his clients better reports than they deserved (blogged about here last year).
Manufacturing false information in support of a plea of mitigation is a form of attempting to pervert the course of justice. In R v Bailey  2 Cr App R (S) 47, the accused invented a story of a dead wife and child to get the sentencing court's sympathy. He had been given a community order for the original offending, but got two-and-a-half years of jail for the lies.
The accused in Church v R  NSWCCA 149 put more preparation into her false claim that she was battling cancer. Several months prior to standing for sentence for an assault, she shaved her head and took to wearing a bandanna, and let it be known around her community that she had been diagnosed with the disease. When she claimed to be attending chemotherapy appointments she would go elsewhere. Her community took up a collection for her medical expenses, and she accepted the money.
The magistrate told the accused when sentencing her for the assault that, if not for the cancer, she would be going to jail. After the ruse was discovered, the sentencing judge for the perversion of justice charge agreed that this should have been so. She was sentenced to imprisonment for a total term of 2 years 8 months for the attempt to pervert justice, with a non-parole period of 1 year 4 months.
She appealed the term, arguing (amongst other things) the sentencing judge had erred by considering the penalty the appellant might have received if not for the lie. This was rejected.
Button J [McClellan CJ and Price J agreeing] at 23:
I consider that it was an important part of assessing the objective seriousness of the public justice offence for the sentencing Judge to determine the effect that the offence had on the proceedings before the Magistrate. By way of illustration, identical lies on oath ("I did not see my neighbour drive his car that day") may be vastly different in seriousness, depending upon whether what was at stake was a fine for driving whilst disqualified, or many years in gaol for murder. I do not perceive error in the sentencing Judge undertaking that task.
The Court of Criminal Appeal found that the sentencing judge was determining what ought to have happened, not what would have happened (which could only be speculative). It regarded the sentence as stern, but not manifestly excessive.