Contraptor

V-track is mcmaster #1582T26 - $4/ft unhardened. Hardened is about twice as much. It's the smallest size.
AL bar is 3/4" x 5/16", mcmaster #1614T533 - almost $5/ft. This comes up to about $9/ft, not including fabrication, which I think is too expensive.

Pre-drilled V-track is available here: http://www.qbcbearings.com/BuyRFQ/LinearB_Track_VG_P.htm# (BK#COM-T107250D), but it's pretty steep $13/ft unhardened or $17/ft hardened. Plus they have very limited quantities.

AL 5/16" bar could probably be swapped for 1/4" bar ($0.75/ft at Onlinemetals), and something 1/16" thick under it, but then pre-drilled track may not work since the holes are centered in its inner wall which is 5/16" tall.

Working on V-rails for contrapteur too.

The idea is to make something lasercuttable ( or extrudable if needed ). Picture :

The goal is to have the same functionality as the makerslide rail : 

but using the aluminium angle as support ( or anything with 20mm dimensions ), and a plastic part as the rail.

The holes and shape of the plastic rail can be lasercut directly. But for the "rail" part ( the angles at each side ), you need a special holder that will hold the parts at 45 degrees angle inside the lasercutter.
About material, I'll use acrylic for the first prototypes as that's what I've under hand, but Acetal is probably better in the range of lasercuttable plastics.

For price, with acrylic it should cost almost nothing, and with acetal, less than 1$/feet ( not including the aluminium angle ).

More on that when the makerslide kits from the kickstarter project ship.

I like that idea of the V-rail. I think acrylic should work for prototyping, Delrin/Acetal could work long term as it's fairly hard plastic. How are you planning to align the rail between 2 opposite 45 degree cuts?

Aluminum extrusion like that would be flat so easier to punch (though still added cost comparing to just an extrusion like T-slot). I also backed Makerslide, looking forward to play with it… will probably need to get some metric extrusions.

For cutting I'll have to design a mount, that's going to be quite a lot of work.
Basically, the rails will be held by the holes, so as long as the mount does not move, the rails should be cut right.
Aligning the mount is probably going to be painful too :)

If you want metric extrusions, I can send you some contrapteur :) ( the sample I already have here is not good enough to sell, but is good enough to use ).