Different Prices in different countries

Many reasons, including but not limited to monetary exchange rates, local regulations, local taxes, costs of shipping, overall costs of doing business, etc. The iPhones themselves, other than any necessary changes due to different cell frequencies and standards, will be identical.

The United States does not have a federal sales tax.

Any US prices don’t include any local sales taxes.

UK prices includes VAT.

Not sure about Canada.

Also as pointed out by Varjak, international exchange rates against the dollar vary.