i paid 750 for a long block lq4 out of a 2005 denali. it only had 20k miles on it. got the p/s bracket and pump, water pump, fan clutch, a/c bracket. coil packs, flywheel.
just look around. your wrecking yards will be the best place to look. or just check on here or even craigslist. 99-2000 6.0 are iron block/iron heads... 01 - 07 (classic) are iron block/aluminum heads, 07 newer are alu heads/ alu block (i think...)
depending on how you want to build the motor... iron block would be your best bet. with the 6.0 block, you can swap the ls3 heads and intake assembly onto it just by bolting it on.
short block with decent rotating assemblies go for 350 +, long blocks can be found from 500+ (just besure to figure out the year the motor is from or you might run into problems if you want to swap a 4l60 versus a 4l80....
cams are your friend!!!!!