wouldnt' say under 1k honestly... cheapest adjustable shocks i could find were the strange engineering shocks/struts, at 400 shipped from summit, then the wheels and tires for the rear alone are 600, then the tune will be what 250+ for your area ( local cheapest is 300, you pay for what you get however ).
so if you were talking fairly stock mustang GT with none of the listed mods i just said your already at 1250 at min without heads or cams, heads and intake manifold can cost any where between 40-200 ( used-new ) and heads can be any where between 150-350 ( used ) cams are 450-650 depending on the ones you wanted + another 200 i think for valve springs. ( dont run cams without aftermarket valve springs PLEASE ).
so now your lookin at 1900 at minimum not including fluids, head bolt kit ( 120 from steeda i think ) , 200 or so if you buy the tools to degree the cams, 30 bucks to rent a valve spring compressor..
Mind you... this is if YOU do all the work