I imagine temperature, barometric pressure, and relative humidity does have an affect on HP/Torque, but does anyone know how much difference it makes?

Like sea level at 75 degrees with 80% humidity versus mile high driving at 40 degrees and 30% humidity?

 

Yes, but the effect is not as great on turbocharged engines.
Most dyno run sheets are adjusted to standard day conditions, a universal standard used to trim jet engines, also used to standardize dyno runs. It's been a while, but I think SDC's are 59 degrees, 0 percent humidity, 29.92 inches barometric pressure. Any jet jock will remember those numbers and can correct me if wrong. To answer your question completely, I'll look up the correction factors and post when I can find them, but I've seen the factors in Bosch's engineering manual and in SAE publications.

 

Bajarider is correct and "standard" temp goes up 3 degrees for each 1000 foot altitude. So if your at 6000 feet on an 80 degree day, your at over 10000 ft "density altitude". Your performance is limited by the turbos ability to compress the air adequately staying within temperature ranges. So even a turbo'd engine will begin to "suck"

 

I wonder how much HP difference there is though from sea level to mile high level?