You can also save money by using recycled or repurposed items as molds. Rather than buying a new smaller mold, you may actually have something in your recycle bin that will serve to make a small batch of soap. All kinds of things work to make a pound or so of soap (3 to 6 bars of soap per pound depending on how large or small you cut your bars.)
For example: I use a wooden recipe box as a one-pound mold. I also have used many a small cardboard box in which various products are packaged; sometimes I just grab them off the shelf in the store when they empty of product & they cost me nothing. 1-quart drink containers (the waxed paperboard cartons) work very well, either standing tall & cut shorter or turned on their side. Various plastic food containers work quite well as soap molds. Some of these items have more rigid sides, but even the ones with a bit more flexibility can be supported at the sides with pretty much anything you may have on hand. Obviously cardboard need to be lined (I use freezer paper, but you can use parchment paper or even plastic bags.)