I am assuming you have a date field for each item , if that is the case you could use the event after update of the price field and enter me.datefieldname =date() . If this is not clear please say so and i will give more details .
Regards
if you are creating a new record when you input the new price then you can do it in your tbl. just set default to Date(). or if you would like to do it in a form ie txtDateInput then you can also change the default to date() in the properties. you coould even use Now() which would give the exact date and time you enetered the new price.