extravagance

As nouns the difference between economy and extravagance

is that economy is effective management of the resources of a community or system while extravagance is excessive or superfluous expenditure of money.

As an adjective economy

is cheap to run; using minimal resources; representing good value for money.

As nouns the difference between extravagance and excrescency

is that extravagance is excessive or superfluous expenditure of money while excrescency is an excrescent state or condition; the quality or fact of growing out of something; abnormal or excessive development; an example of this.