Bongs are a wonderful thing — Although they can be expensive, they don't have to be. Many bongs on the cheaper side of things are made of thin glass, silicone, or acrylic. Since glass and silicone tend to be a little costlier, and often much more fragile, acrylic is a really good, cheap alternative to glassware. A stroll through the internet can give you a great multitude of affordable acrylic wonders.
A good example is this 7" Acrylic Water Pipe with Grinder priced at under $15, this product is all in one, serving as both the grinder itself and the bong. At the base, the bong is fitted with a 2" grinder before giving way into the chamber where the water is held. From there, the bong is equipped with a stainless-steel bowl and stem piece. A carb is located on the back of the piece and the bong is decorated with marijuana leaves to accent the color of the acrylic.
Another good example is a 10" Classic Acrylic Water Pipe which can be picked up online for just over $15. This product comes in many styles and colors, and is equipped with a slide bowl for a carb-less smoking experience. This style of bong is very simplistic, but that doesn't discount the product's efficiency. From the bottom up, it has a stable flat base, which it sits on proudly, waiting to be purchased.
Finally, there is the gas mask compatible Acrylic Water Pipe that can be picked up for just under $30. This bong is very unique, with an accordion shaped chamber, a solid stainless-steel bowl stem, and a carb on the back. This bong is meant to be used with a gas mask, however if funds are tight this is a fun little alternative.
Be adventurous, look online, there are many hidden gems throughout the world wide web. These were all acrylic, however, you can still find many both silicone and glass alternatives.