-Buy online from Ebay and Amazon
You can find a ton of great options for sensory toys that aren't the special education "name brands". I have bought disco balls on Ebay for under $15, light shows on Amazon for under $20, foam and weighted balls for under $6 on Ebay.
-Hit the garage sales
You can find trampolines, bins or tubs for sensory bins, kid toys that make noise, CD players, and a variety of balls for so cheap at garage sales! I found a miniature trampoline for $10 and a bumpy ball and weighted ball for only $3!
-Buy cheap glow-in-the-dark puff paint or paint
Painting any object with glow-in-the-dark paint makes anything cool. I use glow-in-the-dark puff paint to decorate the sensory board, balls, and to put stars on the ceiling.
-Use what you already have (kitchen, lights)
Use items from your kitchen (noodles, rice, popcorn) to fill sensory bins. They are all cheap and you likely already have some on hand. Pull out the Christmas lights you only use 5 weeks a year and let your kiddos explore! If you have a PowerLink, you can plug the lights in and students can even turn the lights on and off! Use old stuffed animals, sweaters/sweatshirts, straws, felt, and pipe cleaners for sensory boards.