Honorarium?

In most cases, fees are flexible and could be up for discussion. I have two blog posts on this topic, the first is more theoretical, and the second is more practical.

A number of factors play into the final honorarium a mohel will receive for his services - details of each follow below.
Expenses - travel, supplies, time
Medical care - he is a trained professional, providing a very important service - in a very delicate area, and he is caring for your son
Ceremony - the style he brings to the service that gives you and your guests the experience you were looking for everyone to have

Expenses - will typically run in the area of $200 (a little more or less depending on how far you live) because you are factoring in the mohel's multiple visits (before, for and after the bris), and his spending time on the road getting to you. (obviously if he is flown in, airfare would likely be considerably more)
Medical care - a doctor friend of mine recently told me that a hospital can bill an insurance company up to $3000 for a circumcision. Obviously a hospital, with all its equipment and staff, has many more expenses. And if the baby is put under general anesthesia (circs on babies three months and up will do this), we can understand. Most mohels are not physicians and do not take insurance, but the skill involved and care for the baby is worth a few hundred dollars.
Ceremony - is purely subjective. It boils down to what it is worth for you, and what style you prefer. Some people like a grandiose show, some like the mohel to create a mood that eases any tension in the room, some prefer a 'cut to the chase and get it over with' approach, and some like to make it an educational experience. Only you can put a value on the style you prefer.

Your financial situation may also play a role in determining what you can afford.

Taking everything into consideration, Parents will typically offer an honorarium between $360 and $1000. (updated 2011)

My policy is to accept what you can offer, without questions, and to say "Thank you."