depends on the brand and style of tire, but with a 6 I would do it even without neccesity just to widen your stance and lower your center of gravity. as for the price, I dunno. I want some though so I'd also like to find some cheap ones.
I have a Rough Country lift, but only the 3". I noticed a big difference in stability, rounding corners, etc, after I added the Spidertrax. I definately recommend them with any lift.