All depends on the year, mileage, what all is included, etc etc. I just sold a complete LS1 with all the accessories, harness, and computer with 32k miles for $2600. 6spds usually go for $1500ish depending on what's all included, mileage, etc. I've seen them go for less, and I've seen them go for much more. The same goes for LS1s.
If your on a budget but really want a 6spd you can save a couple hundred and pick up a 5.3 instead. It'll have just a tad less hp than an LS1, but you'll find much better prices on them.