I answered this very same question in another thread. Here is a copy of my answer:
Get out to Home Depot/Lowes and start pricing out material costs for all items of possible repair on a rehab. You will need a list prepeared so that you can fill in the costs for your area. I uploaded to BP mine for all to use. Go here:
Simply fill in the costs after spending a great deal of time reseraching prices at the box stores. They also have install prices (although they are much higher than you can get by hiring outside of Lowes/HD). Then go to as many REO properties as you can and practice your skill set.
Here is some more info that should help you:
Break up the home into sections - Kitchen, baths, flooring, paint, stucco, landscaping, plumbing (including fixtures), electrical (including fixtures), HVAC, interior doors, door casings/baseboards, windows/sliders/front door, roof, foundation, misc., and garage.
Once you know what the common costs are for each area for a specific size home, you can see how easy it will become to walk in and walk out 15 minutes later with a rehab figure in your head.
For instance, I know that a full gut and remodel including new toilet, new vanity, mirror, sink, glass door enclosure, stone/tile on shower walls, etc will run me $2500 per bathroom. So if I walk into a REO deal with 2 baths, I simply add into my head $5k for the 2 bathrooms and move on to the next section. i also know it costs me around $5k to paint in and out a 1500 sq. ft home. I have these basic figures locked in my head so when I identify sections in each home needing repair, it is simply to calculate in my head.
Hope this helps.