toString() cost

Do you assume that toString() on any given object has a low cost? I do. Is that assumption valid? If it has a high cost should that normally be changed? What are valid reasons to make a toString() method with a high cost?

Dynamic/static language

What if a language allow both static and dynamic types. That might allow the best of both worlds. i.e.:
String str = "Hello";
var temp = str;
temp = 10;

1. Would that be possible?
2. Would that be beneficial?