For unknown artists I prefer the $10-20 price range pencil / inked. At this past Heroes Con I purchased a Bryan Mon
sketch for $20 and he was done in around an hour. Pencilled, inked, colored - and he does his art standing up! Eric Henson
did a headshot for $10 inked in two hours.
The higher your price, the closer you get to getting a sketch from a "pro" (meaning someone who is working in the industry as a known commodity). $100 for a sketch from Joe Bob Whozit or Keron Grant? I'll go with Keron Grant for that amount.
I've had pros put me on a list and come back the next day for my sketch.
An hour to two for a con sketch seems average to me.
Phil Jimenez takes about. . . 30 minutes for an inked head sketch that's around two inches tall.
Evan Dorkin whipped out a Milk & Cheese, inked, and colored in about 15 minutes.
Alan Davis churned out a penciled head shot for me in a crazy amount of time. Maybe 10 minutes.
You want to set parameters for your con sketches. Then practice churning those out to get quick at it. Most fans will be happy with a dynamic figure for a guy, or a sexy figure for a female.
I find that during fast layouts first then going back to ink them goes quickly. Also if you're inking, I pencil loose, then do most of my drawing with the micron / copic / pitt pens. Stuff like this.
Laid them out in about an hour. Finished them each up in around 30-45 minutes. The Soranik took longer because I wanted to horse with ink. Mask. Throw ink down. Throw grays in. Throw whites in. Throw more blacks in.