Worth? A buck or two if you're lucky since it's not really that significant of a "cud" AND its on a well circulated quarter. Kind of like pricing the difference between a 1% Clip and a 20% Clip. Significant clip sizes equate to significant price differences. The grade of the piece also is worked into the equation as a 5% Clip in AU58 will have less value than a 5% Clip in MS64. However, since you were asked, ask him for $3.00.
People love state quarter errors. I'd say in the range of 5-10 bucks depending on how many bidders you get going for it.
Would an error coin like this be worth more in the future and worth keeping until then? Also, could someone help grade it for me? I've got more pictures.