you widen the search and look for comparable neighborhoods (ie you wouldn't compare a neighborhood where the homes are $100,000 value against $400,000 homes, you would find another neighborhood where the homes are the homes are the $400,000). You can also look at the percentage that homes have gone up in an area over a time period. Becareful that you don't get locked into just comparing square footage, without looking at amenities, or updates, or floorplans (there's a lot of square footage that is lost in a traditional floorplan). You also can go back several years to see what the neighborhood was going for (granted things have stabilized or gone down in many areas, not continued to climb, but that will give you a starting point for where they were at). if all the homes are custom homes, that presents another issue, since you can't use what the neighbors home sold for to determine what a custom home is worth. talk to a good local realtor, even if nothing is selling, they will still know what the value in the area is.
Appraisers have a set of guidelines that they use that are more "exacting" than the general guidelines that Realtors use, if you need something that is very detailed.