It's always a smart idea to invest in a proper sports bra, especially if you are busty. I know they are sometimes expensive, but it will remove some of the pain in running (the rest of the pain usually results from not being in shape or increasing your stamina).
As I am extremely busty, I just find it easier and less expensive to not do any sort of extreme sports. And I do try to invest in a proper sports bra, it's just that most sports bras don't work for my specific bra size unless I get them tailored.
IM FAILING THESE CLASSES OH
Good Lord my boobs hurt.