How to keep a record of changes to a custom field?

Question

I have a custom post type that hold real estate listings and I need to figure out a way to keep a record of the price of the property is listed for. For example a user lists a property for sale and then the user edits the property listing and changes the price. I need to show the price before and the new price and the date the price was changed. The price value is stored in a custom field. Doe anyone have any idea of how I can begin to tackle this issue.

I basically need to keep a record of a posts custom field and the date it was changed. If i can get an idea of how to code this I can post the code here for anyone who might find this useful.

Thanks