I found these plastic measuring cups at Superstore, the set of 4 was only $1.00.
They're a little deep for bath bombs and I probably wouldn't fill them all the way for soaps either. I think I filled them about 1/2 way up, that's after I had pressed the bath bomb mixture down really well with my fingers.
I guess the way I've stacked them here you can't really see the contrast in sizes but they are definitely four different sized bath bombs produced by using these cups as molds. Not great for pricing individually if you are selling them but great for gift baskets, the variety if sizes add more visual interest. And really, where can you find a 4 cavity mold for $1.00? Thanks for looking!