Despite it being a cobbled together piece, I have somewhere north of $7500 in my 360, and it doesn't make anywhere close to 580 hp. I paid $0 for the block, maybe $100 for the heads, and I think I got the intake for $150. But, I'd estimate easily $7500 for the valve train, the rotating assembly, (I have probably $1500 into a cast 360 crank. Don't ask.), the cost of squaring, decking, and torque plating the block, 1 sleeve, plus carb, ignition, etc.
Now, I saved a bunch by doing all my own assembly work. Proper hand assembly is pricey. If I was wanting to build a 550 hp 360, from scratch, ready to drop in the car, I would expect to be forking over at least $15-20K.