I love my Dad, he is awesome and made me a soap cutter. I emailed him a link I found online here. He had most if not all of the materials on hand so it was fairly inexpensive, but I think if you went to a big box store it would cost around $50.
Here is a picture of my cutter
It has a guitar wire strung from the bottom (underneath) to the top to cut the soap. It is a bit tricky to get the wire just snug enough to make a straight cut and sometimes it breaks. I've been playing around trying to figure out how to string it properly and I resorted to pulling out my Father-in-law's old guitar and stared at the wires for a bit to figure something out. If you have a guitar or can get your hands on one, look at the way the wires are knotted at the end so you can still turn the screw and not break it. Seems to be working well.
Here are a few pictures of him in the process of making it....this is him putting the table legs on the cutter.
The table with the legs and the side cutter guide but without the top
Attaching the top, which will hold the block with the wires (See the Stained Glass on his workbench in the background...isn't it beautiful!)
Me all smiley while he is working hard!
Another picture of the top...he used Popsicle sticks to separate the two top piece to make room for the wire to run between.
Running the wire from bottom (below) to top
Wrapping the wire tight
And my first cut!
Another shot if it in use