John Maynard Keynes, a member of the turn of the twentieth century Bloomsbury Group and father of Keynesian economics was vociferous and influential during his day, and only more so afterwards in the field of economics and international regulations. It is possible to find truly rare articles of his estate (such as a fine collection of his letters in the War period 1939-41) but they would understandable command a high price; tens of thousands of dollars at auction.
For the earnest collector, however there is still hope, it would be advisable to invest in some of Keynes's less sought after work, but still highly regarded as well as highly influential. Consider instead such works as his The Economics Consequences of the Peace (Harcourt, Brace: New York 1920) which was influential in predicting World War Two (as a result of the reparations that Germany was paying as a consequence of World War One). While it is not a very rare volume, it is still possible to obtain a first edition American printing, cloth bound in blue and with a very attractive Theodore Marburg bookplate for around five to six hundred dollars.
Similar to this volume above, consider also Keynes's A Treatise on Probability which is a mathematical classic, and has been used as a recommended textbook since it was published in 1921 by MacMillian and Co. Like the above volume, it is a ubiquitous book, but a true first edition of the same which is in a good, fine, or near perfect condition is a rare find. Many of these first editions suffer the ravages of time and student life; with stains, annotations, owner's marginalia or even reconstructed spines and covers! A very fine first edition (with dust jacket) of the treatise would easily fetch near five to eight hundred dollars at auction today.
Luckily for us collectors, Keynes was a prolific writer, and one who received some success in his own lifetime. That means that there are still volumes out there and available to collect of his Treatise on Money (1930, MacMillan and Co.) as well as The General Theory of Employment… (1936, Harcourt, Brace) both of which are influential and highly recognized works, indicating that a true first edition with original dust jacket and in good condition could fetch in the region of hundreds of dollars, if not more!
Keynes, John Maynard. The Economic Consequences of the Peace. Harcourt, Brace and Howe, New York (1920). Blue cloth boards, matching dates of 1920 on the title and copyright pages, dust jacket with30th Thousand on the front panel.
Keynes, John Maynard. A Treatise on Probability. MacMilan, London (1921). Dark brown cloth boards with double blind rules to top and bottom, gilt lettering on spine, errata slip tipped in plus 2 pages of publishers advertisements.
Keynes, John Maynard. A Revision of the Treaty. Being a Sequel to the Economic Consequences of the Peace. MacMilan, London (1922). Brown printed dust wrapper. 6 pages of publishers ads at rear.
Keynes, John Maynard. A Treatise on money. MacMilan, London (1930). First American edition. 2 volumes.