I always sell my bikes before they need new tires to avoid the cost!
While there are cheap tires in far off asian countries. Here in the good
old USA it generally costs $150 min to change a 2 wheeled vehicle tire.
Be glad you didn't buy a Vespa it's $300 or more to change one tire on
some of those!
I had an old HondaCH80 and tried to change the rear tire on it. I bought
the tire myself to save some money. Well I couldn't get the rear tire off the
rear drive so I took it to the dealer. They charged me $100 to change it!!
After I paid I asked how the got it off and they wouldn't tell me! I never
went back to that shop...
Some bikes are easier than others to change the tires and you need some
tools to do it so it's not clear what to do. I'd try a few dealers, they are bound
to be hungry and might give you a lower price just to stay busy...