Coins and paper money

And you won't get far because condition is often the most import issue. Take it to a coin shopand ask them to appraise it, then ask what they would give for it.

real value, is what someone is willing to pay for it.

book values are often highest retail prices which a seller seldom see's.

The average $2 bill might get about $3-5 each on eBay, usually several of them sold together, but tens of thousands of them don't sell at all. People want rare and or uncirculated ones.

THe real value is what someone is willing to give you for it at any given time... and that's always subject to change. As this is still legal, valid currency that is still in circulation. Its value is not less than face value.