Here's a new extrusion and V-roller system that might provide a more accurate and/or cheaper linear motion system than what was available in 2015.
http://blog.inventables.com/p/makerslide.html
https://www.inventables.com/technologies/makerslide
I haven't checked to see if the Inventables part is compatible with 8020 or the REV Extrusions, but I'm sure something can be rigged up if necessary.
(I don't work for Inventables, I just saw an interesting press release)