For local shows/national shows at smaller venues, anywhere from $5 to $7 I'd consider acceptable.
For any shows where tickets are getting into the $100+ range, it should be a flat fee of 10% face value. Ticket face value is $125, the fee should be $12.50.
And this is why I love living near the box office and having the power to pay nothing for fees.
Sep. 12 - Saxon, Armored Saint & MindMaze
Sep. 13 - Scorpions & Queensryche
Nov. 20 - Whitford/St. Holmes
Nov. 24 - King Diamond