Real estate sales data is generally publically available, and because real estate is taxed there are public assessments of its value.
The idea of "comparable" works well for real estate, but has the opposite result for domain names. If there are twenty different people who have similar domain names, that is a different value situation from one guy who has a hole bunch of variants in one basket.
For example, if "business.com" is worth X, then is "bigbusiness.com" comparable? What about businesses.com - hey that's more than one business, right?
You want to know the value of anything? Put it on eBay.