How tight a Resolution are you looking for?
How repeatable?
One-off projects - or many repeats of the same design?
(please note I leave "accuracy out of these questions! )
A few simple details would be helpfull in helping identifying the type of slide system your looking for. A few ideas off the top...Not arranged in any particular order.
For example:
1. THK or similar linear rail systems (weiscarver - et al)-ranging from very expensive to less expensive new and sometimes available on EBay - many post throughout the 'zone on their merits.
2. Thompson rail (rod) systems - slightly less than the above - lots of these on the web and can get through EBay too
3. Drill rod in a DIY application less than 2 above - may take some skills and tools.
4. Aluminium extrusion and slide systems - combines the slide with the structure (80/20 etc systems). Expense includes both the structure AND the rail - so gotta figure the merit a bit differently.
5. Sliding rail (drawer style) or Garage Door bits - big and maybe untried - but might be very cheap.
6. Back iron pipe and skate bearings. Heaps of these systems on the site and the web.
7. All sorts of other combos - like a piece of ply or mdf routed and combinesd with bearings to roll along side - gota use your imagination the further youget from the "norm".
Hope that helps out your search - and the quest! Sorry I havent got a "cost per linear unit per mm of resolution"! Wouldn't that be nice eh?
BTW - don't forget to include the length and widths you'll loose (height too) in the system due to the components - their not zero width!
Cheers - Jim