The most direct answer to your question is that there are condos and townhomes in each of those areas that are excellent values. There are also units that were purchased for inflated prices during the "boom" where the owners owe more than they are worth and are therefore over-priced or not able to negotiate enough to be considered a good value under current conditions.
Each area has it's own "feel" or lifestyle. Brookhaven and Dunwoody are more accessible to Marta, Sandy Springs is generally a little less accessible depending on where you are. All 3 areas have older units as well as new or newer units. Lise also mentioned conversions - which are generally apartments that have converted into condos or townhomes, these can be a good value, but be sure your Realtor provides you with FACTS about the areas and recent sales of any unit or complex you are interested in. I always tell my clients that I am there to provide them with facts to make an informed decision.
I'm not sure if you are currently in Atlanta and familiar with these areas, but they are all growing areas for the most part. Sandy Springs has recently become it's own city and has pretty major plans for a more walkable, mixed use (retail stores and residential homes together) type of feel. Brookhaven is certainly nice and building some mixed use now. Dunwoody is a little bit of an older area, but it's also growing.
Another option to consider if you are looking for an area where you can walk to retail, restaurants and more would be the midtown Atlanta area, the area around Peachtree Street between 17th St. and Ponce de Leon. There are a lot of high-rises here as well as 3 or 4 level units with more of a "garden" feel.
I have been working with a number of people lately in Sandy Springs, Dunwoody and Midtown looking at condos/townhomes and I have recent sales data for specific buildings as well as a good general knowledge of the available units in the areas. Brookhaven I know well, but have not sold any condo units in lately.
Here's a few links you may want to peruse:
Best of luck to you! I hope some of this information helps.
Please let me know if I can help!