I just sent someone a quote and I wrote "...On average, I bid around 65 cents a square foot for basements and wall repairs. Without seeing photos or visiting the site, a good estimate should be 80x4x8x0.75=$1920. That's without materials. As I'm unaware of your job's location, out of town projects can cost me significantly more in expenses so I quote them a bit higher."
Now that's as 'bout as fair's it gets; as good a solid free drywall taping quote as I've ever seen.