Hi,
If your x and y are lon/lat coordinates, all that's needed for animation is a simple javascript increment to get the z value (prices?) for a given location over time. If you're mapping for every single town and city in the us, data handled in the browser could be huge (hundreds of megs) and should then be handled server-side. If you're mapping for major cities it could be fine. That can then be styled for each polygon representing city boundaries. This could be done with OpenLayers. The real problem is :
1- getting the boundary data itself free of charge (google API isn't free if you're going to have over 10k requests on it).
2 - for each coordinate, a topological function with JSTS lib has to be called to find what polygon the coordinate belongs to unless.....you have for each x,y value a primary key value such that could be tied to US Census Bureau data (zip code tabulation areas for example).
A simpler approach could to plot data on coordinates with a symbolizer (a square, a circle, or whatever)...no polygons, no topological functions, just plain plotting on x/y and styling on z (timelapsed)
Anyways, I'm bidding because I think I can be of some help if you can provide boundary data in a well-known text, geojson, kml or shapefile format.