Well, the most expensive thing I've lost were some chinese coins. Yeah not expensive money wise, but more as like... interest wise. Anyways, years ago a friend gave them to me when one of my mums friends owned their own store. My mum gave me a thread to put the coins through and war it as a necklace, one day at school (was like..the 4th grade or something) the thread came loose during recess and lost'em in the sand... didn't find out until after school ended.
