“The time of Christians is the price with which they purchase eternity.”

Source: The Principles of Political Economy and Taxation (1821) (Third Edition), Chapter V, On Wages, p. 52

It was not by gold or by silver, but by labour, that all the wealth of the world was originally purchased; and its value, to those who possess it, and who want to exchange it for some new productions, is precisely equal to the quantity of labour which it can enable them to purchase or command.
Source: The Wealth of Nations (1776), Book I, Chapter V, p. 38.

Source: The Genteel Tradition in American Philosophy (1911), p. 58

Often attributed to Jefferson, no original source for this has been found in his writings, and the earliest established source for similar remarks are those of John Philpot Curran in a speech upon the Right of Election (1790), published in Speeches on the late very interesting State trials (1808):
: "It is the common fate of the indolent to see their rights become a prey to the active. The condition upon which God hath given liberty to man is eternal vigilance; which condition if he break, servitude is at once the consequence of his crime and the punishment of his guilt."

*In a biography of Major General James Jackson published in 1809, author Thomas Charlton wrote that one of the obligations of biographers of famous people is

:"fastening upon the minds of the American people the belief, that 'the price of liberty is eternal vigilance' " (in Thomas Usher Pulaski Charlton, The life of Major General James Jackson https://books.google.com.br/books?id=cEcSAAAAYAAJ&pg=PA85&redir_esc=y#v=onepage&q&f=false; F.Randolph, & Co., 1809, p. 85).
Misattributed
Variant: "Eternal vigilance is the price of liberty; power is ever stealing from the many to the few" (from a speech by Wendell Phillips at the Massachusetts Anti-Slavery Society on January 28, 1852; quoted by John Morley, ed., The Fortnightly https://books.google.com.br/books?id=VfjRAAAAMAAJ&pg=PA67&lpg=PA67&dq=%E2%80%9CEternal+vigilance+is+the+price+of+liberty.%E2%80%9D+phillips+speech+anti-slavery&source=bl&ots=H2f8ckIw9o&sig=EukDrduBdK-oQSeY_Gf-VFQ6M54&hl=en&ei=SaxmTN-0H4P98AbioIi0BA&sa=X&oi=book_result&ct=result&redir_esc=y#v=onepage&q=%E2%80%9CEternal%20vigilance%20is%20the%20price%20of%20liberty.%E2%80%9D%20phillips%20speech%20anti-slavery&f=false, Volume VIII, Chapman and Hall, 1870, p. 67).

Things That Have Interested Me, 2nd series (1923), "Secret Trials"

This has been attributed to Marshall, and he might have used the phrase, but earlier uses exist:
There is an imperialism that deserves all honor and respect — an imperialism of service in the discharge of great duties. But with too many it is the sense of domination and aggrandisement, the glorification of power. The price of peace is eternal vigilance.
Leonard H. Courtney as quoted in The Life Of Lord Courtney (I920) by G. P. Gooch
Courtney's statement however is probably derived from an earlier statement with several variants:
The price of freedom is eternal vigilance.
The price of liberty is eternal vigilance.
Eternal vigilance is the price of liberty.
These have often been attributed to Thomas Jefferson, but also Thomas Paine, Abraham Lincoln, and many others; Alfred Denning in The Road to Justice (1988) states that the phrase originated in a statement of Irish orator John Philpot Curran in 1790: "It is the common fate of the indolent to see their rights become a prey to the active. The condition upon which God hath given liberty to man is eternal vigilance."
Misattributed

The Emperor's Old Clothes
Context: [About PL/I] At first I hoped that such a technically unsound project would collapse but I soon realized it was doomed to success. Almost anything in software can be implemented, sold, and even used given enough determination. There is nothing a mere scientist can say that will stand against the flood of a hundred million dollars. But there is one quality that cannot be purchased in this way — and that is reliability. The price of reliability is the pursuit of the utmost simplicity. It is a price which the very rich find most hard to pay.

[Chaturvedi, Pradeep, Women and Food Security: Role of Panchayats, http://books.google.com/books?id=IuKV5ak57asC&pg=PA46, 1 January 2002, Concept Publishing Company, 978-81-7022-873-8, 46–]

1770s, "Give me liberty, or give me death!" (1775)
Context: It is vain, sir, to extenuate the matter. Gentlemen may cry, Peace, peace! But there is no peace. The war is actually begun! The next gale that sweeps from the north will bring to our ears the clash of resounding arms! Our brethren are already in the field! Why stand we here idle? What is it that gentlemen wish? What would they have? Is life so dear or peace so sweet as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!