Gare du Nord (Paris) at night

Definitely take a cab to the hotel. We were in Gare du Nord in the daytime and there were several police around. You may even ask one of them to escort you to the taxi. Just be careful. Ask the taxi drive the cost to hotel before you get in.

Before you get off the train, have no more than about 30 Euros handy. Have the other in your moeny belt along with your credit cards.

We found the taxis were fairly resonable in Paris.

Take the legal taxis that are at the Taxi stand, do not take any of the private ones. A Taxi Pariseine is a metered vehicle, you will not be ripped off. You will however pay more for a night rate, and they will legally charge you for extra luggage(one euro per bag over one) .
When you get off the Eurostar just start looking for the signs for "Taxi". Find taxi stand and get in line.

Although you can ask the taxi driver what the approximate cost would be, it is charged by the meter so just pay what the meter says.

Make sure you have Euros with you before leaving London, as the ATMs at Gare du Nord were all out of order (looked to be permanently so) when I went through that station. I'm not sure if the exchange offices are open that late or not. Even if they are, their fees are too high. Also, avoid the homeless-looking men that want to walk you to an available taxi. They'll walk you to a taxi, but they'll expect you to pay them something for their trouble. They're scary looking in the daytime...I definitely would avoid them at night. Stick with the legitimate taxi line, and you'll be fine. I liked the suggestion about seeing if an officer will escort you to the taxi line/stand.