YouTube member Sunny01 shares a a simple DIY roller bearing slider most people can build out. A few T-shaped joining brackets on each side, some carefully spaced bearings on top and bottom allow the carrier to ride over the PVC pipes. If PVC isn't your thing, there's no reason you can't redesign around some lightweight copper piping. [Thanks Sunny]. Judging from the build and how it maintains itself on the rails, you'll need a minimum of 12 skate bearings. Normally these bearings come in packs of 8.
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
Igus is the manufacturer of the popular Drylin W Linear Guide rail. Linear guide rails are seen in robotics and manufacturing type machines and associated with lube and grease, not good for cameras. This design uses 'Dry Bearings' so it was natural that this rail exploded onto the DSLR Video scene as one of the most widely used rails for DIY camera sliders AND by actual retailers of Camera sliders. From what many are saying, the early adopters for this rail call this the 'ZaZa Slider' and you'll find a group on Vimeo from people making it from Igus rails.
You can see this same basic rail design in Glidetrack, Kessler, and others. When I first created my DIY Camera slider, I had to call IGUS in Ireland, figure out the exact parts I needed, and have it shipped from overseas. Looks like so many camera manufacturers have been using their rail, Igus is now hip to DSLR video, and they are now posting an item at Amazon.com specifically as a 'Camera slider'. The nice thing in this image is the new side clamp on the carrier designed to lock the carrier in place. I was using old hand clamps to keep mine from shifting as I was transporting.
You can check out my DIY Slider in this video to see what this whole thing looks like put together. There was a bit of drilling needed to get the rail onto the tripod, and a bit of drilling to get the Fluid head onto the Igus Carrier, but very easy material to work with. Take my advice and don't get 4 feet of rail. It's nice and fun, but soooo long it's akward to carry it around. You'll be better off with 2 feet, 3 feet MAX. Well thanks to Igus, it looks like they are making things a bit more streamlined for us DIY'ers interested in building our own camera slider with a straight purchase to the complete set. Below is a link to the most popular one they are saying, but here's a link to More Igus Camera Slider Rails.
Modular DryLin® W linear guides ensure a smooth, lubrication-free gliding motion for camera slider systems. Used by manufacturers of high-quality, complete systems, as well as individual DIY video enthusiasts seeking individual components for camera dollies and camera slider assemblies.
Our most popular camera slider size. Includes 1,000mm-long guide rail (39.4") with 9 holes evenly spaced and a 100mm-long carriage plate. Carriage plate features 4 low-friction plastic bearings for smooth and quiet operation, as well as extra bolts to prevent misalignment.