Is it ethical to imprison non-dangerous white-collar criminals to set an example, or should they merely suffer financial penalties?

The description of "non-dangerous white-collar criminals" itself is somewhat problematic. Many such criminals have indeed been dangerous to their victims. The victims of such criminals can be ruined financially. One of Madoff's victims committed suicide. Victims of identity theft, revenge porn, and other technologically enabled crimes can have their lives ruined. White-collar crime can include corruption, which weakens democratic institutions and leads to oligarchy and increasing inequality in which the very wealthy feel increasingly entitled to act with impunity, as though they are above the law. White-collar criminals also have relatively high rates of recidivism.
Merely assessing financial penalties for white collar crimes sends the message that a poor person who steals food should be imprisoned while a rich person who steals millions of dollars should just pay a fine. This is fundamentally unjust. The message it sends to society is that it is a crime to be poor and hungry and steal food but being rich and greedy and stealing additional money one does not actually need is merely a minor miscalculation.
In terms of imprisonment, for all criminals the issue is the same, namely whether sending them to prison is likely to protect society from them and prevent future crime. While no one should be sent to prison as "an example" as that implies imprisonment for some reason other than justice being applied to the individual case, the two other criteria of protection and prevention apply just as much to people who steal millions from bank accounts as to pickpockets who lift wallets. "White-collar" criminals present clear dangers to our society and letting them off with minor fines does not protect society from their actions nor deter them from re-offending.
https://www.nytimes.com/2016/11/08/business/dealbook/determining-a-punishment-that-fits-the-crime.html