The 2006 GMC Sierra 1500 is available as a 2-door, 3-seat regular side. Starting at $25,945, the 2006 GMC Sierra 1500 SL 4x4 Regular Cab 8' Box is powered by a 4.8L , 8 -cylinder engine mated to an automatic transmission that returns an EPA estimated 15-mpg in the city and 19-mpg on the highway. The top of the line 2006GMCSierra 1500Work Truck 4x4 Extended Cab 6' Box , priced at $ 00 , is powered by a 5.3L , 8 - cylinder engine mated to an automatic transmission that returns an EPA estimated 15-mpg in the city and 19-mpg on the highway.