One of the things that inspires me in finding or figuring out how to make cheap therapy tool alternatives is by going and getting magazines from therapy stores that I find on the internet. I look for the sensory input area I need for with my son and then find what products are being used commercially and see if I can find material that will give a simular sensation for a cheaper price. Examples of this would include the intertube swing which I made for $25 rather than spending $125. A scooter board you can get for around $18+ dollars which you can make for less than 1/2 that price. (That is my son's next project.) Using an old sheet filled with old pillows or packing peanuts for a crash pad or a 2X4 to make a balance beam are also cheap things you can probably find around for free.
Some things can be bought at stores for a fraction of the price. Balls, bins, stretchy toys, blown in musical instruments, bubbles, ropes are just a few things I have found at dollar stores. (There will be a post on this later I hope.)
Just make sure what you purchase will work for your child. For instance, I know what my son's meltdowns are like and that bins from the dollar store are easily broken in the middle of a rage. It is more cost effective and safer (broken sharp pieces of plastic) for me to spend a buck or two more for a less destuctable bin or option that then cheaper product.