What city?

Most Anglo-American jurisdictions make real estate sales prices a matter of public record, though you'll have to look at specific properties. To get an overview for an entire city, perhaps a "market basket" of representative properties would be a good approach.

Assessed value for property tax purposes is another possibility, but it's difficult to separate land from improvements, and many jurisdictions play funny games with assessed values that aren't related to actual sales prices.

Deeds are a matter of public record. Mortgages are recorded alongside deeds. The Los Altos or the county's Recorder of Deeds (or similar name) will have a series of books.

The giant books will either be tract indexes, showing all the prior owners of a certain property, or grantor/grantee index, showing in one book all deeds organized by the owner and in the other by the recipient. These books will list the deed numbers to certain properties. You then can look the deeds up in the deed books. If there was a mortgage, it will be right after the deed in the book.

Some cities have separate mortgage records too. You can simply match the names of the grantees to the names in the mortgage books.

If you want to find a specific property, go to the auditor's office and get the property card. It'll list the current owner of the property. Let's say you want to find out how much a certain property sold for in 1964. You'd find the current owner. Hopefully, his deed says, "A, the grantor, who received title from C in a deed recorded as 111, 222 in the Los Altos Recorder's office,grants to B, the grantee, in fee simple absolute, title the property described as________." Then you go to deed 111,222 (the first number is volume and the second is page). That deed will hopefully direct you to the previous deed. When you go back far enough, simply find the associated mortgage.

Sometimes the sale price will be on the deed, but more often than not it will say, "For $10 and other valuable consideration."