The port is in the fixed price zone (I think it's currently CAD38.00 for a taxi and $49.50 for a limo). When you exit the airport there is a taxi/limo stand. Ask the guy to make sure he agrees that it is in the fixed price zone. It should take about 30 minutes at the legal speed limit.
They will take USD but will give you change in CAD and a horrible exchange rate. CAD38 is about USD31 at the moment, but expect him to tell you USD35 at least. Some will tell you USD38 but that includes the tip. They are quite generous with your tip. (Canadians generally tip $2 for a trip, but maybe a bit extra for the luggage).
If you are going to need CAD in any case, you can go upstairs at the airport where they have a number of ATMs from different banks including the Bank of Montreal, Desjardins Co-op and the RBC (Royal Bank of Canada). Just ask for a "guichet" (geesheeh) and people will point you in the right direction. It will be cheaper than trying to get CAD in the US unless you live in a border town.