Eleanor Roosevelt earned 75K during the first year of FDRs presidential term from giving lectures and a syndicated column.
The first lady IS a job, and has been since at least Dolley Madison (who served under both Jefferson and James).
http://en.wikipedia.org/wiki/Dolley_Madison#In_Washington_1801-1817
"Dolley Madison worked with the architect Benjamin Henry Latrobe to furnish the White House, the first official residence built for the president of the United States. In addition, as Jefferson was a widower, she sometimes served as his First Lady for official ceremonial functions."
http://www.firstladies.org/biographies/firstladies.aspx?biography=4
At those receptions and dinners which the widowed President [Jefferson] felt necessitated a female co-host, he asked Dolley Madison to aide him. While she was not a presidential wife or in any way given an official designation, her exposure to the political and diplomatic figures who were guests of the President, as well as to the general public who came to meet him, provided her with a lengthy experience as a White House hostess. Notably, she also took a large public role in the fundraising effort that supported the exploration of the Louisiana Territory by explorers Lewis and Clark. These eight years of fore-knowledge and opportunity to consciously create her own public persona were the crucial factor that enabled her to shape what was only a marital relationship to the President into a genuine public role that was soon called "Presidentress" by some chroniclers.
She wasn't the first First Lady, but she was probably the one who made it into a political and powerful position at a time when women pretty much had zero political power.
And for good measure:
Here's Dolley's account of the burning of Washington:
http://www.nationalcenter.org/WashingtonBurning1814.html
Here's a historical essay on it:
http://www.whitehousehistory.org/whha_classroom/classroom_documents-1812.html
It's up to each first lady to determine what she wants to do with the position (some are more hands on than others), but being first lady IS their job.