The one that you walk around with a scanner, as cvb says, is worth it to me. ONLY for $25+. I'm insulted lately by Market Force's playing with fees and pretending that they are going up. I've seen these shops now at $10 and they used to be $33!
So anyway, you shouldn't do them for less than $25. Expect them to take one hour to one and half hours, depending on how efficient the person is that you walk the store with.
Also watch out for the managers, I do one store here where he thinks he is clever and whispers to his staff to run ahead of us and correct signs and price tags etc. He doesn't fool me though, but he's the only one I've ever caught doing deceptive things.
Otherwise I love getting paid $25 for what takes me all told about 1 1/2 hrs.