How to make web form that comapre numbers?

How to make a real estate site and enable users web form, where they can type in, for example, min and max values of square or cubic meters. In order to do that I should add fields that "pick up" numeric values, and tell Drupal to do simple math and compare numbers (Example of that kind of form pay antention to Property details section).
But Drupal, although offers numeric, or decimal fields, treats information inside like textual data.
I have been told there is Computed Field module , but I am not sure what php skills is required to play with it.
Did anyone of you cope with this problem personally, or have any suggestion, anyway?
Thanks in advance!