Been helping a buddy build a FF Cobra for the past 3 yrs. I'm not sure what he has invested in it and since he's gotten married since the project started I doubt you'd get him to post an answer in a public forum, but I do know it's on the high side of $25k and counting, and the body work is still yet to do. My guess is he's going to be in the mid-30's after paint.
There's a lot more to it than the price of the kit. The doner car (for things like engine, trans, diff, heater, gas tank, brake Mcylinder), misc items like gauges, steering linkages, coolant tanks, belts, hoses, thousands of rivets, cleco's, drill bits, tubing flaring kit (some of the mating fittings have no off-the-shelf adapters), driveshaft (takes a pretty short one -and don't forget the safety loop), electrical wiring components, seat tracks, shoulder belts, fuel lines, battery....and you probably better budget for some sheet aluminum to remake a few of the parts that don't seem to fit just right from the "factory". I have heard they are doing a better job with the kits now than they did then so it might go a little smoother.
Then there's the basic stuff like a welder, drill press, good tap & die set, a super low jack...A good background in metal fab work and auto building/restoration would be helpful as well.
And on the subject of junk yard engines - you can go that route, save a few bucks up front. But what happens when that engine starts smoking, losing water, making funny noises or the tranny starts acting up...a few months or a few miles down the road. He went kinda wild and stuffed a (salvaged) blown 4.6 into his and we've already had to pull it back out twice and there's no way under the sun you get that motor out with the body still on. Not sure if you put in a 302, you may be able to squeeze that one in and out. But the last thing you want to do once the project is "done" is to have to pull it back apart to rebuild an engine or tranny. And have to explain that to your wife, if you have one.
Nothing wrong with getting a doner from the boneyard, but do yourself a big favor and rebuild or have someone who knows what they are doing rebuild the engine, tranny & diff before you bolt them into the car. It could be your cheapest way out and save a lot of headaches down the road.
But it's fun!