To “give away the store” is to give away a substantial amount in a transaction. For example, a sports team desperate to acquire a superstar player might “give away the store” in a trade—surrendering many players, drafts picks and cash. The phrase is often given the slight qualification of “practically/almost giving away the store.” A much older and similar expression is to give away (or pay) a “king’s ransom.”
 
The term “give away the store” has appeared in newspaper advertisements since at least 1951.
 
   
Wiktionary: give away the store
Alternative forms
give the store away
Verb
to give away the store
1. (idiomatic) To transact, trade, or negotiate badly, by paying, providing, or conceding too much to the other party.
 
Merriam-Webster Dictionary
king’s ransom noun
Definition of KING’S RANSOM
: a very large sum of money
First Known Use of KING’S RANSOM
circa 1590
 
1 November 1951, The Register News (Mt. Vernon, IL), pg. 17, col. 1 ad:
WE’RE GONNA PRACTICALLY GIVE AWAY THE STORE IN JUST 2 DAYs
(Illinois Brokerage—ed.)
 
10 September 1953, Winona (MN) Republican-Herald, pg. 2, col. 1 ad:
I can’t be responsible for what he does for the next few days (he’s practically giving away the entire store) until he gets some rest.
(Cortland’s—ed.)
 
29 August 1954, The Oregonian (Portland, OR), pg. 13, col. 3 ad:
while George is away we’re practically giving away the store during Picard’s AUGUST MONTH-END CLEARANCE
 
27 May 1956, Charleston (WV) Gazette, pg. 36, col. 1 ad:
“Just wanna say that I’m willing to do anything short of giving away the store, to give you the best deals you’ll ever get.”
(Joe from Joe’s Surplus—ed.)
   
11 February 1966, The Plain Dealer (Cleveland, OH), pg. 25, col. 2 ad:
We Would Give Away The Store…But Frankly We Can’t Afford It.
(Heinen’s fine foods—ed.)
 
24 August 1975, Springfield (MA) Sunday Republican, “Throwing Badly—Plunkett” by Carlo Imelio, pg. 44, col. 2:
Veteran quarterback Roman Gabriel, for whom the Eagles practically gave away the store to get from the Los Angeles Rams, has failed to consistently move the offense—and backup man Mike Boryla hasn’t performed wonders, either.
 
OCLC WorldCat record
Are we giving away the store to home health agencies?
Author: EH Anderson
Edition/Format:  ArticleView all editions and formats
Publication: Journal of professional nursing : official journal of the American Association of Colleges of Nursing, 1985 Nov-Dec; 1(6): 319-20
Other Databases: MEDLINE
 
OCLC WorldCat record
Taking care of business : 101 ways to keep your customers coming back without whining, groveling or giving away the store
Author: Dick Schaaf; Ron Zemke
Publisher: Minneapolis, MN : Lakewood Publications, 1991.
Edition/Format:  Book : English
 
OCLC WorldCat record
GIVING AWAY THE STORE Deep discounting could pervade retailing for the rest of the year
Edition/Format:  ArticleView all editions and formats
Publication: BUSINESS WEEK -NEW YORK- no. 3835, (June 2, 2003): 51-52
Other Databases: British Library Serials