I am building a wall from concrete retention wall bricks. I need to cut one brick in half so it can fit in the wall. The solutions here are too expensive, I only need to cut one brick. Is there any low-cost way to cut a brick? I'd prefer not to spend more than $20 USD on a tool to cut it, so perhaps there is a hand tool that can cut through that?
OK, so you don't want to spend a lot of money. Get a 1/8" or 3/16" masonry bit and drill a series of holes along your cut line. Drill as deep and straight as possible. Have some water near by to keep the bit cool. It will take a while to do this and do is slow to not overheat the drill. Flip the stone over and drill more holes if the first side holes didn't go through. Then get a screwdriver (I know, not the right tool for the job) or small chisel, place it in the first hole in line with the cut line and give it a whack with a hammer, but not real hard. Go to the other edge and do the same thing. Go back and forth to the rest of the holes doing the same thing, working the screwdriver/chisel further down the holes until the stone breaks along the cut line. I have done this when no options were immediately available (Gunflint Lake, Minn) but it took a lot of time... and I had to change my name to Joshua the stone cutter.
To a large degree it depends on where you need to cut the block. These blocks are somewhat brittle -- they'll break quite readily where the sides intersect, but it'll be quite a trick to get a break in a controlled way somewhere in the middle of a cell. Not impossible to do with a hammer and chisel, but I can tell you it's going to be challenging. You'll go through several blocks trying to get it right. If you support the inside of the block, for example by putting lumber through the hole, that'll help.
Your other option to try to get close to $20USD is tool rental -- realistically, though, your total cost may be more like $60 even just to rent a saw.
Then there's the "classifieds tool rental program." Find a used machine in a local classified ads listing, buy and use it, and put it back up for sale when you're done.