I think it depends on how many mods ... Mine was $700 but I only had a coilover front end and disc brakes plus the usual stuff like headlights, seatbelts etc.
I would budget on about $1k.
There are lots of various Chevs and Holdens running around (legally) with LS1/2/3s .. Maybe jump on one of those forums for some first hand advice.
Cheers,
Frank.