Vimeo member J.G. Pasterjak shows how you can take a few pieces of angled iron, 8 skate wheels, and some miscellaneous nuts and bolts to whip up a 'Ladder Dolly' inspired slider design. He doesn't really mention the words Ladder Dolly, but this design has basically been around for quite a while. Cheap ladder dollies although will still run you upwards of $700 dollars, and that's still missing the $100+ dollar Ladder. His design if being selective of where you choose your parts, in my own head, i'm estimating still falls under the $45 dollar mark and you can get away with just about anything for rails. You can find his DIY video here: DIY Inexpensive Camera Slider.
Below i've embedded a video from Hague showcasing the layout of wheels used for a Ladder Dolly Design. You can find more information about the Hague here: Video Camera Ladder Dolly System
If you're not the DIY type and want a cheap rail slider, the best deal so far and most popular is the IGUS kit from Amazon. You can see my IGUS DIY build here: http://cheesycam.com/diy-camera-slider/. Some IGUS systems cost around $80 dollars, but I suggest going for the wider rail that's already pre-drilled and just about ready to go. Found here: IGUS Linear Motion System for Camera Sliders - pre-drilled version