From: https://history.stackexchange.com/quest ... a-revoluti
A Russian revolution caused by the Bolsheviks was most definitely the goal of the Germans when they allowed Lenin to pass through their lands. Germany wished to undermine, or end, the Russian war effort and sending Lenin back was done for that purpose.
If true, who came up with the idea and was there any consideration that a communist Russia could eventually be a threat to Germany?
The historian Richard Pipes writes in his book The Russian Revolution that based on disclosed German papers made available after WWII the German Foreign Secretary at the time of Lenin's passage Richard von Kühlmann was either the person that came up with the idea, or at the bare minimum signed off on the idea of letting Lenin pass through German lands to return to Russia. Lenin was even decried as a "German agent" when he finally returned.
As to whether there was any consideration of a future threat from a communist Russia, the answer seems to be no for two reasons. The first reason is that Germany was primarily concerned with the demands of World War I. Surviving the war was paramount. The second reason is that, according to former professor Albert L. Weeks, Lenin was a Germanophile. Weeks argues that Lenin viewed Germany as the central linchpin to an eventual proletarian revolution. Weeks further discusses the close relations enjoyed between the two nations after World War I, and how this relationship extended back into the 19th century. The relationship still exists today, see e.g. pipelines, or former German politicians heading Russian companies.
How much did the arrangement cost the Germans, money-wise?
According to Pipes, relying on numbers from Eduard Bernstein, the German government sent "more than 50 million deutsche marks in gold" from 1917 to 1918 to help the Bolsheviks establish and hold power. In 1917 US Dollars, 50 million marks would mean $9,041,591 — adjusting for inflation this equals about $172,910,538 in 2017 US Dollars.
The investment was substantial, and at least with respect to achieving the goal of ending Russian involvement in World War I, the investment paid off.
The German government was not a supporter of Lenin's ideology. Lenin published various pamphlets and books throughout his life making it clear he believed in abolishing the German monarchy. Their goal wasn't to have Lenin succeed with communism, but to incite a civil war to finish off the Russian Czar, or at least force him to sign a treaty for peace.
More information on the Bolsheviks and Soviet Communism:
(PDF) The Secret Behind Communism - David Duke
https://web.archive.org/web/20190507011 ... munism.pdf
Jews in the NKVD of Stalin's Soviet Union
Winston Churchill - Zionism vs Bolshevism