if I choose to shop in the USA and return to Canada within 24 hours, what taxes and duties would I pay?

PST and GST are calculated depending on the value of the goods and the province you are resident in. You may also be assessed duty depending on what the goods are, and where they were made. For example a cotton shirt made in China, would have duty charged on the component that originated outside the US. To avoid duties, look for the "made in USA" label. If you are internet shopping, you may also be charged brokerage by the carrier, ie. UPS. The brokerage fee is compensation for the carrier having to figure out and charge the duties and taxes. Some internet retailers do the brokerage themselves by charging Canadian taxes and duties with the item. This gives you the most predictable price.

How much do you spend?

I think that it depends on how much money you spend.

please refer to the thread below. Basically its PST / RST and GST / HST. They may throw duties at you.