In item-post.php, check this line:
<span class="float-left"><?php ItemForm::price_input_text(); ?></span>
And change it like this:
<span class="float-left"><?php ItemForm::price_input_text("0"); ?></span>
When a user publish an ad, will have as its starting price of € 0 / $ 0 / £ 0.
As a result, at will change this line in Osclass / helpers / hItems.php:
if ($ price == 0) return osc_apply_filter ('item_price_zero', __ ('the words you prefere if an user dosn't change the base price €0'));
Users will thus "forced" or enter a price, or if they do not make no price, they will automatically price € 0 / $ 0 / £ 0