"My two cents" is a rather small contribution except when compared with "a penny for your thoughts."  Actually, two cents was the amount that you could get for turning in a Coke bottle in the 50's.  However, my brother (and a buddy) turned in bottles as a kid and found a 1955 double-strike penny worth hundreds of dollars.  So it doesn't always work.
My 2 cents.. why is it called so?