i would insert the data differently into the dates table, otherwise you are constantly adding columns (property_1, property_2... property_70) each time you add a property. the way below means you only add the property once to the properties table, then each property gets its own row for each date.
you might think that would mean a lot of rows (365 x 70 in the case of 70 properties, but mysql handles that without blinking - make sure you put an index on the property_id column in the dates table)
property_id | price | date_field
1 | 60.00 | 2012-12-13
2 | 70.00 | 2012-12-13
1 | 60.00 | 2012-12-14
2 | 75.00 | 2012-12-14
1 | 65.00 | 2012-12-15
1 | 65.00 | 2012-12-16
1 | 70.00 | 2012-12-17
(the order of insertion doesn't matter).
it sounds like you'll need a custom form for inserting special dates. something like:
Property: dropdown menu: <option value="1">Property 1</option> etc
Price: price field
processing the form, you need to capture the 'from' and 'to' fields, work out all the days between and put them in an array, then update the price for that property on those days.
a quick search gave me this function for getting the days between two dates: [edrackham.com
once you have the array of dates, implode(',', $array_dates) to give you the IN() clause for the SQL
UPDATE table_dates SET price = '$_POST['price']' WHERE date_field IN ($imploded_array_dates) AND property_id = $_POST['property_id']
the IN () clause won't work in the code above as dates need to be escaped. it should look like:
IN ('2012-12-13', 2012-12-14'). remember to escape all the POST'ED input as well for security.