around $6000, but that motor with more boost will support 800+hp will factory internals.
Any DOHC lincoln motor with new rods and pistons (~$1000) and some boost will make that much HP. Either way, you are spending about 7-8K for a 700hp engine(assuming new parts) - disclaimer, this is just a guess on part prices, but it’s fairly close. 
How much is it to build a lsx? It may be a little bit cheaper but not much.