1. If you purchase an unlocked iphone and then go to either canada or mexico... could you then buy a canadian sim card or mexican sim card and get that iphone to work in that country and then use a canadian or mexican monthly cell phone plan?
Yes. The carrier must use the frequencies supported by the iPhone, but in general the answer is yes for Canada and Mexico.
2. If yes, how much generally does it cost to get that sim card in canada or mexico? Also, once you leave that country and come back to the usa, if you want to use t mobile, you would have no issues switching a canadian or mexican sim card and getting a tmobile sim card and then get a tmobile sim card right?
It depends wildly on the carrier and the kind of service you want. Pre-paid SIM cards are typically around $10, plus the service charge on top of that. $30-$40 for a month of service is common in the US. It varies by country and even location in the country.
The phone will work fine on T-Mobile when you come home. Just put your sim card back in and you'll be back in business with your own phone number.
3. Because if so, this seems like the best idea for someone who wants to have an iphone around the world but for someone that say is in canada 6 months then usa a few months then back and forth?
If you're spending a lot of time in both countries it would make sense to have prepaid service in both countries. Then you can switch as appropriate when you move between them. You may end up with a lot of overlapping months, though, depending on when you move and when plans start and stop. This won't be as cheap as staying put.
4. Im curious about the cost as well. How much does it cost to make an iphone to be able to be used with a certain carrier such as a canadian cell service... then change it back to tmobile in the usa? How much does it cost each time?
This sounds a lot like question #2.
5. Also i assume if you have an iphone unlocked and got it to work with tmobile... then go to canada or mexico... you could make your cell work in other country simply by switching that sim card? Or would it require unlocking each time you go from country to country?
Nope, unlocked is unlocked. You'll be able to swap SIM cards at will.
6. Does anyone know what canadian carriers that an iphone unlocked bought in the usa would be compatible? What about mexico? I assume most won't know in mexico but curious about canadian. Also i assume an unlocked iphone bought in the usa to be used in canada... the plans would be same plans as if you bought an iphone in canada?