From Thursday (today) up until Monday October 26th most Fallout games will be on sale at the Steam store. Games range anywhere from $3.30 to $9.99 on the Steam store.
If you want to try any of the Fallout games before Fallout 4 comes out on November 10th then now is the time to strike.
If you want a full list of the games here is a link to the steam store.
Fallout New Vegas and Fallout 3 are less than 10 dollars combined in the steam store which by it self would be the best deal. Or if you played both of those and you want to play some of the older games Fallout 1,2 and Fallout Tactics are on sale in a package for under ten dollars.