Don't Go Too Low
It's probably the most expensive dress you will ever own in your life, and you want to hire someone to hem it that understands that and treats the dress carefully. If you're just keeping your eye on the price, you risk dropping your valuable dress off at a place with low-quality service.
Be Wary of Scams
Wedding dress retailers will charge the most to hem, since they know their shoppers have a higher budget than most and are willing to pay more for quality and convenience; they might charge upwards of $300 to $400. However, your local seamstress may be able to offer a lesser price for equal-quality service.
Shop around before you commit to a place to do your dress alterations. You shouldn't pay more than $200 for a simple hemming job, as long as you feel comfortable with your chosen seamstress.