Two Maps I Would Like to See
The data to produce that information almost certainly exist online, since there are extensive databases of webbed real estate listings. Run some regressions on that data and you can use the results to estimate how much the same house costs in different places. The results will also tell you how the price of a house depends on its area, lot size, age, etc. Do it right and the potential buyer can input a description of the house he wants and get estimates of how much it would cost in any of the places he is considering moving to. He can input different house descriptions, compare prices, and use the information to help him decide just how much house he wants to buy and where. The same approach could be used for rental prices. And it could be done not city by city–prices within a single city can vary a lot–but neighborhood by neighborhood.
Housing prices are not the only thing you want to know. Another consideration is the crime rate–relevant not only in deciding where to live but where and when to take a walk. The map for that information lets you set a category of crime (burglary, mugging, assault), a time of day, and see a map of the relevant area with crime rates shown by color, running from bright red for the highest to dark blue for the lowest.