I used to wear foam forms. (They were inexpensive.) They were considerably lighter than real, home-grown breasts. I've seen ads for weighted forms, primarily for women who've had one breast removed so that the two sides will weigh about the same.
As I have started growing my own, I find the weight distribution different from wearing forms - they sit on the body differently. Even if real breasts weigh more, I think you will find that they are more comfortable because of how they attach to the body. JMHO
- Kate