Extopia - good advice, but I always found that asking EAs for selling prices was met with "Sorry we can't reveal that information, but it was excess of asking price" (no, really??).
Personally, the only way I found of actually gauging selling price was to get in and follow the bidding on properties. Ring up about every property and ask for the current offer. Eventually, you'll be told it's gone Sale Agreed, so you cann make a good guess as to the selling price.