I can't vouch for the accuracy of the following:
Two Cents Plain
In a letter received a few days ago, our correspondent Michael Raynor reported that he had spent the better part of two days at a conference with twenty-four accountants, and had put his time to good use by dreaming up questions for this column. On the off chance that I might somehow curry favor with some accountants and thus parlay my answers into a tax deduction, today I tackle another of Mr. Raynor's queries. "Why do people only contribute their 'two cents worth' when making a contribution to conversation or debate?", Mr. Raynor wonders, "Inflation aside, I don't think two cents ever really amounted to much purchasing power."
Speaking as someone who grew up a few short blocks from a first-rate penny- candy emporium, I beg to differ -- two cents per day, applied in the form of liquorice whips and gumballs, can buy you a lifetime of dental adventure.
In any case, the whole point of "my two cents worth," which originated in the late 19th century, is that it is a faux-modest, self-deprecating tactic used to disarm your audience before you announce your opinion. This is especially important in the event that your opinion turns out to be idiotic, in which case you can always claim that you warned your listeners in advance that your opinion was next to worthless. The phrase has long since become a cliche, and its use can be especially grating when the person announcing the arrival of his "two cents worth" is a doctor or lawyer (or a dentist, come to think of it) charging you two hundred dollars an hour. Don't get me started.
"Two cents" or "two-center" has been a slang synonym for "very cheap" since the middle of the 19th century, when the cheapest cigar available was literally a "two- center." The U.S. Treasury Department actually issued a two-cent coin in 1864, which was, incidentally, the first U.S. coin to bear the motto "In God We Trust." The government, evidently feeling frisky in a monetary sort of way, also issued coins in three-cent and twenty-cent denominations during the same period.
Found at
http://www.word-detective.com/back-k.html.