Going to Canada to buy a car is quite easy....since you will be paying in USD you get the exchange rate saving to start. One USD is worth about $ 1.60 cdn.
Take the MP3 as a example list was 23,700 cdn and I believe is was 18,500 US. 18,500 US = 29,600 cdn, so you can see the savings here.
Now there are other points to consider
You will pay Frieght and PDI say $ 1500 cdn
You will pay taxes ( gst and PST ) 15 % ( you may be able to get the GST back which is 7 % )
So on the MP3
23700 sticker
1500 freight and PDI
3780 taxes
Total $ 28,980 = $ 18112 USD
Now you have a host of things to consider
Insurance is mandatory to buy a car
You will have to pay a licence fee for a trip permit ( 30 bucks )
You will have to pay tax in your home state on the purchase price, duty is 2.5 %. You will have to make sure the warranty is valid in USA.
For more info go here
http://www.customs.ustreas.gov/imp-exp2/informal/car.htm
When all is said and done you will save a few bucks but not a lot.