A D90 should do a great job.
The rail is a little over $200 as described here:http://thedambook.com/smf/index.php?topic=3675.msg20000#msg20000
but you also need the Nikon PS5 or PS6 (ebay?)
Then you need to modify it for so that it can be attached to the MPR113 ($50 or so for the part)
You'll also need a light source.
All in all, cheaper than any comparable scanner, but not as easy to acquire. (Assuming that you can find another use for the D90 and Macro lens)
We do have a limited number of the PS5 and MPR113 ready to go for $200, so you'd be looking at a bit over $400 for the 10 inch rail.