You can enter Rivals mode
via 2 ways, the Community menu or via the Career.
Once in the Rivals menu head to the Top Gear Rivals
section (First right) and the first Rival challenge is called 'A Reasonably Priced Car
' this is the challenge you can use to easily unlock the achievement on, accept the game settings and begin the race upon and completing your lap the achievement will unlock.Note: If you havnt already unlocked Rivals Shootout in Forza Motorsport 4 you can also unlock this along the way assuming you beat the opponent's time posted.
Alternatively you can setup a Quick race (Accessed via the Free Race
menu) on the Top Gear Full Circuit
and unlock the achievement that way.
Here's a video demonstration of the rivals method.