The most valuable information would be the most current, of course. A sale last week has more validity in helping you determine a purchase price than a sale from six months ago. The problem is that there is no actual record of the sales price until the transaction is completed. The information is not available in the public record because no deed has yet been recorded.
Neither is the information available in the Multiple Listing Service. Once a property is sold, it becomes a "pending sale" and all pricing information is removed from the listing. Prices are not posted until it becomes a "closed sale." This protects the seller in case the transaction falls apart and the property is placed back on the market. It would give an unfair advantage to future potential buyers if they already knew what price the seller had been willing to accept in the past.
However, if a Real Estate Professional has a reason to know the sales price, they can usually find out through professional courtesy. Also, some real estate brokerages post sales information on a transaction board in their office.