if you just wanted no price to show that is easy, just go to the listing filter and select it as a non required item this way they can fill it in or leave it blank ? or add a second listing filter with a drop down or text area ?
I want to be one listing filter ( maybe mixed) , which be required to fill.
Put price or check "Ask for price" .
Then on listing page will be show ads with price and other without price but with text Ask for price.