Hello guys. Thanks for your comments
I'm using classipress 3.0.4 and I made some customizations of my own.
Version 3.0.4, already allowed to create our own forms, so I created a new form with the price field as optional. This will let people post ads without a price value.
Another option is to edit the theme files and change the display options. This requires some knowledge of php and may cause problems if you make a mistake. So please consider using a custom form. If you really wanto to edit it, search the customization forum. I think I saw a couple of examples there.