I looked into this recently in quite some detail. I checked with customs, RIV and every link/ phone # I was provided with.
You are looking at 7% GST and 5.8% duty, cost of the USD exchange and about $400cdn in documentation fees. These %'s are all based on the purchase price, I spoke to a guy at a dealership that imports cars quite often and a "dummy" bill of sale is useful in the regard, ie. make 2 cheques to the seller and 2 bills of sale, use the lesser amount as documentation when bringing the car across. There is also a luxury tax but you just have to make sure you document the car at less than $30000usd i think was the benchmark.
When the car gets to Canada you have to take it for inspection (done at Cdn Tire) believe it or not and you will need airbag stickers, these are available on-line. In some extreme cases, (depending on the inspector) you may have to order new sunvisors from bmw with the stickers intact which are supposedly not removeable....guess they never heard of goof-off. Another thing is to make sure the DRL's are enabled as the cdn dealers don't have access to this encoding in the lighting control module.
BMW's are probably one of the friendliest vehicles to import. There are some other little rules like contacting the border 3 days prior to crossing with the car so they can run a title search in order to make sure the car wasn't stolen.
I would definitely look into buying my future car in the states and bringing it across as I have made many contacts from guys on the boards who would be willing to help scout prospective rides.
I found an '04 m3, smg with nav and 19's vert a couple of months ago with 2800 miles and could have had it shipped here all documents done for just under $67000cdn. Shipping was about $2000 cdn btw. Would have been a great deal but bailed on it as the economics of parting out and selling my car were not really in my favor. I figured this to be a $25000 savings vs buying from the dealership new here and a slightly kicking myself for not doing it, but have found a new love for my car since my tuning issues have been sorted out.
2008 Porsche 911 C4S, gone but not forgotten 2008 E60 ///M5, 2003 E39 ///M5, 2002 E46 330 CIC Supercharged.