Hypothetical 475-500hp 360 build cost.....???

See there's a few different parts to your specs. You want it to make 500hp. you want it to cruise on pump fuel. You want it to have street manners. You want cheap. You can get 500hp from the 5.7 with an original long block. You cannot on the LA 360. So you have to decide first if you want the next gen deal, or the LA deal. In other words, it's a strictly cash thing or it's not. If you have the Hemi long block, use that.
In terms of building horsepower, for 500hp I'd use $14/hp, or $7K for a turn key, everything new but the block and crank LA engine. The Hemi saves you from having to build a new bottom end and saves due to the heads' abilities. That's really the only differences.