This isn't exact, but it gives you a rough indication:
If you convert your Canadian dollars at a bank, you will typically get USD converted at the spot CAD/USD rate (which right now is 1.350) plus ~2.7%, unless you have preferred rates with the bank.
If you convert your Canadian dollars at a local f/x shop, you will typically get USD converted at the spot CAD/USD rate (which right now is 1.350) +/- some amount which is usually better than using a bank.
If you use a typical Canadian credit card, you will typically get charged the USD cost converted at the spot CAD/USD rate (which right now is 1.350) plus a 2.5% f/x fee, plus ~0.4% Visa/MC markup.
If you use a no-f/x fee Canadian credit card (Scotia Passport Visa Infinite, HSBC World Elite MC, Brim MC, Hometrust Visa, Rogers World Elite MC), you will typically get charged the USD cost converted at the spot CAD/USD rate (which right now is 1.350) plus ~0.4% Visa/MC markup.
If you use a USD-denominated Canadian credit card, you will get charged the USD cost, but you still have to find some way to pay that card off off in USD, by converting your CAD to USD (see first two bullets), unless of course you have USD income or have saved up USD.
So, unless you have USD income or have USD savings, the least expensive option would be to use a no-f/x fee Canadian credit card (Scotia Passport Visa Infinite, HSBC World Elite MC, Brim MC, Hometrust Visa, Rogers World Elite MC). To put it in perspective, if your trip costs US$5k, you will save at a minimum $170 by using a no-f/x fee credit card (and that's without even taking into account the value of the points you'll get from the credit card)