Dan, I think prices vary quite a bit across the country, but here's some ballparks from my area (SC)
1) Kitchens...too hard to say since there are so many degrees of finish, but expect to spend a large portion of your rehab budget here
2) For a small bathroom in a typical rental, I'd say 4-6k. That's with nothing too fancy -- you could easily burn 10k if you use nicer tile, fixtures, etc. I just did one about 75 sq ft for about 4k. That included gutting to the studs, joists, re-plumbing everything (tub and toilet were re-located), re-wiring, insulation, moisture-resistant drywall, new window, new basic tub, subfloor, underlayment, fairly cheap tile on floor and tub surround, sink/vanity from lowes, basic toilet, paint, etc. I did most of the work myself. If I had hired it out, I think I would have spent about 7-8k. For a low end rental, using vinyl floor, basic (read: cheap) materials and fixtures, and a lowere grade of workmanship/finish, you could have probably done it for 3-4k. But my experience is that a nice kitchen and bathroom will pay for themselves in increased rent over a fairly short period (as long as the neighborhood will support the rent)
3) Basic vinyl, double pane, decent windows cost from 150-200 apiece as long as you can use standard sizes. Figure 250-350 for custom sizes. Figure about 150-200 apiece to have them installed properly.
4) I think roofing is particularly subject to regional differences, but here you can get a basic shingle roof done on a small (900-1400 ft) house done for 1500-2500.
Hope this helps,
JMac