The 1999 GMC Sierra 1500 is available as a 2-door, 3-seat regular side. Starting at $22,796, the 1999 GMC Sierra 1500 SLE 4x2 Extended Cab Short Box is powered by a 5.0L , 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 1999GMCSierra 1500SLT 4x4 Extended Cab Long Box , priced at $ 27,630 , 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 18-mpg on the highway.