NOTE: Cost operators are specified as being by value or by percent. If
the operator is by value (add, subtract or multiply), this means that cost
modifiers are applied directly according to the value attribute of the
modifier. That is, for a selection of 'AddValue' with a value attribute of 20
on a cost of $120, the modified value would be determined by $120 + $20 = $140.
If the operator is by percent(add, subtract or multiply), this means that cost
modifiers are applied by transforming the value attribute from a percentage to
an actual value . That is, for a selection of 'AddPercent' with a value
attribute of 20 on a cost of $120, the modified value would be determined by
$120 + $(120 * 20/100) = $144.
