My definition:
1. The value of a product is the portion of society's labour
needed to reproduce it.
2. The value of a commodity is either the value of the product
that embodies it, or, in the case of a commodified service,
the direct and indirect portion of society's labour required
to perform it.
3. The exchange value of a commodity is the set of triples
{[q1,u1,c1], [q2, u2, c2], ... etc } where
qi is a rational number
ui is a unit of measure ( grams, litres etc )
ci is some category of use value ( corn, olive oil etc )
for which the commodity will exchange
4. The price of a commodity is the set of pairs
{ [q1, m1], [q2, m2] .... etc }
where qi is a rational number, and mi a unit of
currency.